The occurrence of severe rotavirus gastroenteritis (RVGE) in children under 3 years of age before and after the introduction of rotavirus vaccine was prospectively surveyed in three pediatric clinics in Shibata City, Niigata Prefecture, Japan, during the 2011 and 2012 RVGE epidemic seasons. In this observational study, a significantly lower occurrence of severe RVGE among severe gastroenteritis cases was observed in 2012. The incidence rate of severe RVGE among outpatients in 2012 was significantly lower than that in 2011. Despite the significant reduction in severe RVGE, the results must be interpreted with caution because the surveillance period is short and requires extension to conclude whether the reduction in the incidence of severe RVGE is a direct effect of rotavirus vaccination. Therefore, we will continue the survey to evaluate the impact of vaccination.