Using data from the Japanese Panel Survey of Consumers (JPSC)1994-2006, we examine the effect of child birth on fathers’ wage rates and labor supply in Japan. We also compare effects of fatherhood among different cohorts by dividing the JPSC sample into two birth year cohorts (born in or before 1960 and born after 1960). We find that birth of child significantly increase hourly wage rates by 2.8 percents and annual work by 65 hours. Comparing with results in the U.S. (Lundberg and Rose 2002), the effect of child birth on labor supply is large but the effect on wage rates are relatively small in Japan. We also find that child birth have different impact on labor market outcome between the early and the later cohorts. In the early cohort, birth of child significantly increases wage rates but has no significant effect on labor supply. On the contrary, birth of child does not increases wage rates but significantly increases labor supply in the later cohort. Finally, we examine how gender difference of children matters. Although the impact of gender difference is not so large, the effect of birth of sons is larger than the effect of birth of daughters.