Since the 1920's, there has been a slow fluctuating upward trend in primary sex ratio (ratio of males to females at birth) in Japan. The factors :concerned with the rise of sex ratio have been analyzed by using the vital statistics and other data for the period from 1947-1969. There is no evidence indicating that the age of the mother or the birth order are contributing factors causing an increase in the ratio. The recent decrease in still birth rate suggests a possible relation with the increase of the sex ratio at birth. Provided that the ratio of the number of still births to that of live births were the same in the periods of 1955-64 and of 1965-69, the extent to which the sex ratio at birth in 1955-64 would be modified was calculated. By this calculation, the modified sex ratio at birth in 1955-64 was 106.2, in contrast the actual value was 105.8. The correlation between the sex ratio at birth and some social indices by prefectures has been examined in the respective periods of 1960-64 and 1965-69. For both periods, signifi cant correlation coefficients were obtained between the sex ratio at birth and the rate of hospitalization at delivery. Only in 1960-64, social indices as the percentage of employed persons in primary indust ries to total employees, the "MYNRYOKU" index (synthetic index of socioeconomic indices) and the percentage of women workers to total employees were significantly correlated with the sex ratio at birth by prefectures.