One hundred and twenty five fecal specimens were collected from sporadic cases of acute gastroenteritis in a pediatric clinic in Osaka City, Japan from July 2004 to June 2005 and tested for the presence of rotavirus, norovirus, sapovirus, astrovirus, and adenovirus by RT-multiplex PCR. Among diarrheal viruses detected, norovirus was the most prevalent (19.2%, 24 of 125), followed by group A rotavirus (18.4%, 23 of 125), astrovirus (1.6%, 2 of 125), and adenovirus (0.8%, 1 of 125), respectively. Interestingly, sapovirus infection was identified with high incidence of 17.6% (22 of 125). Sapovirus was subjected to molecular genetic analysis by sequencing. It was found that sapovirus detected in this study was classified into four genotypes (GI/1, GI/4, GI/6, and GI/8), and sapovirus GI/6 was predominant, followed by GI/8 and accounted for 81.8% (18 of 22) and 9.1% (2 of 22), respectively. It was noteworthy that sapovirus GI/6 infection was apparently confined within the period of 5 months (August 2004 through December 2004). This pattern of infection implied the outbreak of sapovirus GI/6 in these patients, which was the first outbreak of acute gastroenteritis attributed to sapovirus in Osaka City. Another interesting feature of the study was the appearance of two novel, naturally occurring recombinant sapoviruses circulating in the Japanese population studied. This report confirmed the presence as well as the importance of sapovirus causing acute gastroenteritis among Japanese infants and children.