A study on the prevalence of antibodies against human astrovirus serotype 1 (HAstV-1) and 2 (HAstV-2) was carried out in 170 serum samples obtained from different age groups in Tokyo and Saitama, Japan. Prevalence of specific IgG antibodies to both HAstVs showed decreasing tendency after birth and reached a minimum at the 3-5 months age group. Then, prevalence of antibodies to both serotypes increased after 6 months old, and nearly reach to 100% by the third year of life. The antibody titer to both HAstVs in sera revealed same pattern of antibody prevalence. The sera collected during epidemic season between January and June, had more higher titer of antibody to HAstVs than sera of non-epidemic season between July and December. Our results indicate that the infection of astrovirus in community may be more frequently than previous reports, and infection mainly occur in the 6-11 months age group.