Norwalk virus is one of the most important diarrheal viruses. Although the virus was discovered in 1970, the genetic analysis has started from 1990. The virus appears in sporadic cases but also in outbreaks in children and adults. The symptoms are vomiting and diarrhea, but rarely are convulsion and others. The molecular epidemiological method is useful to detect the pathogen in environments and hosts. As the results of molecular epidemiology, it was found that the common virus with the identical sequence spreads in the world and the virus with unique sequence spreads in a limited area. In Japan, Norwalk virus infection occurs in early winter in children, while, rotavirus infection in late winter. In addition to food borne transmission, air borne transmission is strongly suggested. Norwalk virus infection is recognized to be higher prevalence following rotavirus infection.