Variations in the amount of Japanese Encephalitis (JE) virus in the field and outbreaks of JE were examined. The causes of changes in outbreaks were examined. The following results were obtained. 1. The level of JE virus estimated by positive rates of HI reaction and the 2-ME reaction of JE of swine tended to decrease from 1972 to 1977, and tended to increase from 1978 to 1980. On the other hand, positive rates of 2-ME reaction in 1978-1980 were higher than those in 1965-1971. 2. The number of JE-patients was more than 50 from 1965 to 1967, thereafter it decreased and no JE-patient was detected from 1971 to 1977. Thereafter an outbreak occurred again. Two definite cases and one suspected case were recognized in 1978, one suspeted case in 1979, and one definite case in 1980. 3. One cause of the decrease in JE-patients is that the size of a pigsty has been increased and sanitary conditions have been improved. 4. Discussion was made that the course of the outbreak of JE-patients from 1965 to 1980 was attributed to the amount of JE virus in the field estimated by antibody reaction in swine, the number of Ct, the arrangement of pigsties, the spread of window setting nets, improvement of nutrition, decrease of overwork and ruralization.