Conducting the immuno-epidemiological study of hemagglutination inhibiting antibody (HI antibody in short) titer in this disease patients and epidemiological study of outbreak of patients in Okayama prefecture noted for this disease prevallence, the following results were obtained. 1) The level of HI antibody titer in the serum of many inhabitants in the southern part in Okayama prefecture increased by J. E. vaccine inoculation to them and revealed the immunity strengthened, of inhabitants, from this disease, although HI antibody in 21% inhabitants in number yet stayed at the value, below 1:10. 2) Morbidity rate of vaccinated inhabitants was (1/2.0) as high as that of non-vaccinated and the difference between the two rates was siginificant within 0.1% level proving the evident effect of vaccination, and the ratio of mortality rate between vaccinated and non-vaccinated was also (1/2.0) showing samely the difference significant. 3) Annual variation in morbidity rate of vaccinated inhibitants paralleled with that in non-vaccinated inhabitants, and the effect of vaccination was especially high in pupils in the southern part of the prefecture and old men in the northern part. 4) Outbreak of J. E. was usually influenced by the number of hazardous mosquitos under various environmental conditions and the number of the vaccinated inhabitants in that region, for the vaccination effect on this disease was not absolute but relative to the ratio between the numbers of the vaccinated and non-vaccinated. 5) It was observed that the higher ratio between the number of vaccinated and non-vaccinated, the lower morbidity rate. 6) Morbidity rate of inhabitants at the age of 5-14 in the southern part of Okayama prefecture became lower after having received vaccination. The rate of receiving vaccination was higher in pupils than in inhabitant of any other stages of age. 7) Morbility rate in the southern part of Okayama prefecture has decreased in recent years and that in the northern part of Okayama prefecture has increased after 1960, and on the other hand, morbility rate of pupils in the former part has markedly decreased and that of the old in the latter part has increased after 1960. It is considered that these above phenomena are due to cleaning up of various environmental conditions by the rapid industrization in the southern part of Okayama prefecture and the superior effect of vaccination.