With the establishment of the multiple labor unions permitted since July, 2011, and subsequently with the bargaining channel unification System applied, it has become possible for workers to freely organize and join the labor unions having mutually different purpose and values in workplaces. According to such a permission of in-company multiple labor unions, the labor-management relations come to produce complicated legal issues, showing the tendency of instability in the relations between the labor and management. In other words, as a legal issue, a worker can organize, form a labor union more easily and also join a labor union with ease, and consequently their withdrawal from a union, organization of a new union and joining of a different union are freely guaranteed. Meanwhile, an employer has been familiarized with one union in one company or established unified working conditions all this while, so their response to the multiple labor unions is linked to tension, causing the labor-management instability. Under the multiple labor unions requiring such tension, imposed on an employer is the obligation to stay neutral without giving preferential treatment to, or discriminating against a specific union in principle. In other words, an employer is required to maintain a neutral attitude towards each labor union, and to equally approve & respect workers’ right to organize as well as not to give discriminative treatment to any labor union irrespective of each labor union’s character, tendency or ideological line. This paper is intending to examine the unfair labor practice under the coexistent multiple labor unions through the cases by selecting its representative type with a critical mind with focus on the supposition that the types of unfair labor practice in case of only one labor union’s existence in one workplace and in case of the multiple labor unions in one workplace could be different. The first type this paper selected is an employer’s discrimination against a labor union or its members free of rationality, i.e. the discriminative treatment in terms of several union activities. In other words, an employer suppresses the anti-corporate union through implicit or explicit support of a union, thus producing discrimination against personnel or economic matters. Second, it’s a matter of refusal of collective bargaining with either of the two unions or a matter of single bargaining channel. In other words, in the unification process of collective bargaining channel, it becomes inevitable for two unions to compete to be bargaining representatives; moreover, it’s because should a union, which obtained the bargaining representative right through a majority union, monopolize the collective bargaining right, the foundations of the other minority union could be threatened. Third, it’s a matter of expense assistance and assistance of union activity to a particular union by inclusively labeling the entire act of interference or sabotage in the activity of securing workers’ labor’s three primary rights as anti-union behavior through an employer’s domination of and interference with a specific union. As mentioned above, here, this paper is to analyze and examine with focus on the unfair labor practice, such as an employer’s discriminative treatment between unions (union members), collective bargaining, and domination & interference, etc. under multiple labor unions.