Since the middle of the 1970's, the major part of reproductive populatoin has been the generation born during the period of rapid fertility decline after the War. Author examined microscopically this trend through the observation of the effects of sibling number on marriage, birth etc., using survey data of 2,034 couples with at least one child younger than 6.25 years old. The main findings are as follows. (1) The effect of sibling number on school career has been robust and negative for both husband and wife and been strengthened for newer cohorts. (2) Residential relation which expresses whether a couple lives with or near their parents is negatively and strongly affected by the number of siblings. The number of siblings of spouse reversely affects it. (3) As for age at marriage, the indirect effects of sibling number is stronger through school career and co-residence with parents than direct effects. Nevertheless if wife is only child, which is assumed to be unadvantageous by the necessity to co-reside with her parents, wife's age at marriage is higher. But this effect has been being attenuated. Co-residence with parents or parent-in-laws has a positive effect on age at marriage for both husband and wife. Psychological cost accompanied by co-residence with parents may raise age at marriage. (4) School career of husband has positive effect on fertility. Indirect effect of sibling number through this can be inferred as negative. Effects through age at marriage are negative for wife and positive for husband. Direct effect of husband's sibling number on fertility is small but positive. These effects of sibling number allow us to speculate that the decrease in number of siblings has been one of the factors affecting the expansion in educational enrollment rate and also one of factors raising age at marriage through school career and co-residence with parents. The decrease in number of siblings is to continue for around 1965 birth cohort. Therefore changes in demographic behavior through the change of sibling number will last until the beginning of the 1990's.
