Using three household surveys, theJapanese Panel Survey of Consumers(JPSC), theWorking Person Survey(WPS), and theSurvey of Adult Skills(PIAAC), this study estimates the returns to postgraduate education in Japan, considering potential self-selection bias. To mitigate the bias, workers' undergraduate majors, types of university, and level of cognitive skills are controlled for. These factors explain 6.3-29.2% of the postgraduate wage premium for women, but at most 10.9% for men. Even after controlling for them, the postgraduate wage premium remains positive and significant, ranging from 16.5 to 23.7% for men and 13.5-26.4% for women.