A survey of consanguineous marriages in Japan was conducted on September 1 in 1983 through questionnaires. The total number of couples studied was 9,225 chosen from six widely different areas of Japan. The rates of first cousin marriages and of total consanguineous marriages for all areas are 1.6% and 3.9%, respectively. The mean inbreeding coefficient is 0.00134 for all areas of Japan. On the other hand, the rate of total consanguineous marriages was ten times higher in Fukue-Shi (7.9%) than in Asahikawa city area (0.78%). The rate of total consanguineous marriages is decreased with the marriage year in Japan, where the rate of first cousin marriages has changed remarkably.