This study investigates that what type of utterances Korean and Japanese high school students use, and how often they are used as well as how honorific and casual expressions are used in terms of honorifics, which is one of the characteristics of the two languages, with a focus on when they express gratitude, apology, refusal and request, which are frequently used in their daily life. It was carried out targeting high school students in Korea and Japan by analyzing the results of the Discourse Completion Test. Through this research, the following results have been revealed. In the speech act of gratitude, high school students in both countries use more direct gratitude utterance to older people who are complete strangers, while the reason-type gratitude or extra-type gratitude utterance is relatively more observed when they talk to elders with whom they are well acquainted. The apology-type gratitude utterance is more observed in Japanese students and reason-type gratitude utterance is more observed in Korean high school students. The characteristics of expressing gratitudes with casual language to the elderly or expressing gratitudes with honorifics to the younger, which is an unprecedented phenomenon, are resulted from the fact that Korean and Japanese languages both have honorifics, which they can choose depending on the other interlocutor. In both countries, it shows that the rate of female students using honorifics to younger people is higher than that of boys, which can be explained by the tradition of women using more courteous language than men in terms of the sociocultural aspects of both countries. The Japanese language is different from the Korean language. Unlike the Korean language, the Japanese language has the linguistic convention of using female language and male language separately. The result can be explained as the influence partially left in Japanese languages. Also, the higher rate of their casual utterance to the elderly shows that the demand of using honorifics to their parents are relatively less strict. To sum up, the use of honorifics displayed in the process of gratitude utterance of adolescents in both countries shows that even though both languages have honorifics as a common feature, they reveal different aspects. In the speech act of apology, even though the majority of high school students use the honorifics in both Korea and Japan, casual utterance is overwhelmingly more used when talking to their older siblings or mother. It shows that most Korean high school students use casual language with their older siblings, while most Japanese high school students use casual language with their mothers. In the apology utterance to the elderly, it is a natural phenomenon that the majority of high school students use honorifics, which is a common feature of both languages. Nevertheless, it shows that they express apologies to their mothers, not by honorifics but by causal language, which results from the intimacy with someone specific. Along with this, it reveals that Japanese students relatively feel free to use casual language with their parents, compared to Korean students. On the other hand, Korean students feel deeper intimacy with siblings compared to Japanese students, and Koreans tend to treat their elder siblings as if they are friends. This comes from the difference of family cultures between the two countries. In terms of the speech act of request, Japanese students use the apology type of request utterance more often than Korean students, who use the reason type of request utterance. The casual type of request utterance to elders is mostly shown in high school students in Japan. In particular, female Japanese students show a higher rate of using the utterance with their fathers, and when it comes to regions, rural students use it even more frequently. It is notable that these characteristics come from honorifics of both languages. Also, it shows that honorifics are spoken by high school students in both countries, and that there is the deeper intimacy between fathers and daughters than that of between fathers and sons. In the speech act of refusal, the reason type of refusal utterance is the most used and male Japanese students generally tend to use the direct type. The apology type is mostly displayed in Japanese students. Especially, female students and rural students show a higher rate respectively. The reason type is overwhelmingly displayed in Korean high school students. In terms of extra-type, the higher rate is shown in Japanese students. In the case of Japanese students, even though the interlocutor is younger, quite a number of students use the honorific type of refusal utterance, especially Japanese girls. In the situation of refusal to elders, a small number of Korean students use the direct type, while Japanese students show an extraordinarily high rate of using this type of refusal expressions. When Korean students use honorifics with their parents, the rural students show a higher rate than those in central areas. These characteristics come from the common features of both languages, where people can choose between honorifics and casual language depending on the interlocutor and this study shows the customs of language and the use of honorifics of high school students in both countries. Also, in the case of the casual type of refusal utterance to their parents, the difference is quite big among the two countries. Japanese girls use the casual utterance more often to their parents than Japanese boys, which shows that the relationship between parents and daughters is much more intimate in Japan and they use much closer and unconstrained expressions. On the basis of the characteristics from the results analyzed above, the conclusions to be drawn here are as follows: Considering gender and region difference, it turns out that the former tells the difference between high school students in Korea and Japan more than the latter does. Japanese high school students mostly use the apology type of utterance, while Korean high school students use the reason type more. The tendency of using casual language with their parents is quite different between Korea and Japanese highschool students, and it is less strict in Japan compared to Korea. In terms of speech act of high school students in both countries, there are quite big differences in genders and regions as well as in cultures. Japanese high school students relatively freely use casual type with elders compared to Korean high school students, and quite often they use honorifics to the younger people in their daily life. Even though both languages have honorifics, which is one of their common features, the use of honorifics shows different aspects. Both languages have lots of similarities in grammatical structures, expressions, and so on, but there are many differences in terms of the culture of family relationship. This study reveals the differences of speech act of high school students in Korea and Japan. Furthermore, this study is quite meaningful in that it suggests basic data in terms of sociocultural aspects in learning the other language.