I investigated the significance of the retail investors' sentiment as a determinant of the IPO underpricing in Japan and quantified the magnitude of the effect of the sentiment.  The IPO underpricing is about 90% on average for firms listed on the emerging small-capital company market from 2004 to 2014 in Japan.  I used the daily market turnover rate in the first five trading days after IPO as a proxy for the sentiment.  The higher turnover indicates the higher sentiment.   I found that one sigma increase (20 percentage points) in the turnover rate increases the IPO underpricing by 26-30% on average and reduces the one-year return after an IPO by about 30%.   Further analyses show that the sentiment, measured by the turnover rate, is decreasing in the share allocated to retail investors, who are presumably susceptible to the sentiment.  This result is consistent with the assumption in the theory by Ljungqvist, Nanda, and Singh (2006) that retail investors' sentiment is decreasing in their holdings of the issued stock.