In this study, the effects of internet book piracy in the case of the Japanese comic book market were examined using direct measurement of product level piracy ratio and a massive deletion project as a natural experiment. Total effect of the piracy is negative to the legitimate sales, but panel regression and difference-in-difference analysis consistently indicated that the effect of piracy is heterogeneous: piracy decreased the legitimate sales of ongoing comics, whereas increased the legitimate sales of completed comics. The latter result is interpreted as follows: piracy reminds consumers of past comics and stimulates sales in that market.