Recently, with the improvements in rapid antigen test methods for viruses and widespread use of high-sensitivity genetic diagnostic methods, the diagnosis rate of the causative pathogen in cases of acute gastroenteritis has improved remarkably. Therefore, in the Soma-Futaba area of Fukushima Prefecture, in which the rotavirus (RV) vaccine was introduced recently, we identified the outbreak trend of viral gastroenteritis using pathologic samples of children hospitalized with acute gastroenteritis, and examined the effect of introduction of the RV vaccine. Over a period of 24 months, between January 2015 and December 2016, we examined children who were admitted to the Pediatrics Department of Soma General Hospital for acute gastroenteritis. The severity of each case was evaluated at the time of admission, and stool or anal swabs were collected as pathologic samples. Identification of the causative pathogens was attempted using rapid diagnostic tests, culture tests, virus isolation methods, and polymerase chain reaction assay techniques. The hospitalized children with acute gastroenteritis were divided into RV-positive and non-RV-positive groups, and the effect of the RV vaccine was evaluated using a test-negative design. During the study period, a total of 238 children were hospitalized with acute gastroenteritis. Presence of RV, norovirus (NV), and/or adenovirus (AdV) was detected in 146 samples (61.9%) by rapid diagnostic tests. In addition, using virus isolation and PCR techniques, other pathogenic viruses were detected in 191 samples (80.3%). In addition, of the 66 RV-positive patients, 8 (12%) had received the RV vaccine, and of the 158 RV negative patients, 67 (42%) had received the RV vaccine. Using the test-negative design, the RV vaccine was determined, after adjustment for gender, age, and presence of siblings/going to a nursery school, to be 81% effective (95% confidence interval : 55.9-91.8). The present study revealed that in most cases, acute gastroenteritis in hospitalized children is caused by a viral infection, and also that the RV vaccine is significantly effective in preventing hospitalization of children due to RV gastroenteritis.