Successful IPO managers are likely to show self-confidence and optimism. Behavioral finance points out that managers'' psychological convenience can influence corporate value by inducing inefficient decision-making. This study examines whether the manager''s over-investment in R&D at the time of IPO is likely to be the manager''s optimistic over-investment from two aspects. First, we examine whether firms with high discretionary R&D expenditures in the IPO year are more delisted after IPO. We also examine whether the positive relationship between discretionary R&D expenses and delisting in the IPO year differs depending on the type of auditor. We select 999 IPO companies newly listed on the KOSDAQ market from 2001 to 2015 as a sample and analyze the hypotheses on these samples. The analysis results are summarized as follows. First, we find that firms that overspent discretionary R&D in the IPO year were delisted more than other firms. These results suggest that discretionary R&D expenses in the IPO year partially reflect the over-optimism of management. Second, a positive relationship between discretionary R&D expenditure and delisting in the year of IPO was found in the sample of Non-Big4 auditors, but no significant relationship was found in the sample of Big4 auditors. These results suggest that Big4 auditors have a greater monitoring effect than Non-Big4 auditors in suppressing the over-optimism of IPO companies. This study additionally performs (i) an analysis using changes in operating performance instead of delisting, (ii) an analysis that controls the profit management effect, and (iii) an analysis that considers the effect of the designated auditor system. The additional analysis results do not show any qualitative difference from the previous results. Overall, this study finds the possibility that inefficient R&D investment due to management''s optimism at the time of IPO may become an additional determinant of delisting. In addition, the monitoring activities of the Big4 auditors with high independence suggest that IPO managers'' optimism is partially reduced.