Gender Wage Gap is still as large as 40 percent on average in Japan, and work interruption is still common even for university graduated females today. The government implemented Equal Employment Opportunity Law of 1985, Child Care Leave Law of 1991 and enhanced the laws numerous times. Nevertheless, the newest National Fertility Survey of 2010 showed no rise in the work continuation of first time mothers after 2005 as compared to first time mothers in 1990's or 1980's. The trend is well contrasted with the U.S. where the work continuation of females with children has risen dramatically in the 80's and where the gender wage gap significantly narrowed. The purpose of this research is to apply innovative empirical research method to compare the gender wage gap in the U.S. and Japan, and to point to the mechanism of labor markets and internal labor markets that create the differences through analysis of internal labor markets and the effect such market have on fertility. I will also take into account the values and norms and provisions of child-care. My main focus will be on the U.S. and Japan, but I hope to add a brief sketch of another country, presumably Australia, or Sweden, to include the country whose wage setting is strongly influenced by unions. I will first apply empirical methods to the micro data of Labor Force Survey of Japan after 1980, as Blau and Kahn(2006),Blundel et al(2007) , Mulligan and Rubinstein（2008）did for the U.S. and U.K. data, and to show to what extent and where the narrowing of gender gap occurred in Japan as compared to the U.S. Such careful scrutiny using elaborate method of econometrics has not been yet made for the Japanese data. The result will be of much interest, especially to check if positive or negative selection into full-time employment occurred in Japan. I presume that negative selection into employment might be found till early 2000's. I will also employ the 21st century Panel Data of Adults conducted by Ministry of Health, Labor and Welfare to analyze different choice of employment on wages and fertility, while controlling for individual unmeasured differences in error term. The results will be compared with the results of U.S. using Current Population Survey and National Longitudinal Survey of Youth. I would like to discuss the results with U.S. labor economists. I will also use national data collected by myself for Japan for females in the age group 26 to 38 in Japan, and web survey collected by myself for U.S. for females in the age group 25 to39 in six cities in the U.S. I will then analyze the differences in the internal labor market practices for hiring, promotion and wage settings for the cause for the differences in two countries. The analysis will made through the interviews to working mothers and to personnel departments at firms. There have been comparative studies on U.S. and Japan with attention to marital relationship such as Funabashi(2006), or on general labor supply model such as Osawa(1994). However, in depth internal labor market mechanism with attention to female workers and the effect on fertility has not been yet made. The result will give rigorous suggestions for policies that enhance career build-up of mothers and decrease opportunity cost of childbirth in Japan.