Seasonal level of hemoagglutinin inhibiting and its 2 -mercaptoethanol sensitive antibody in the sera of swine were researched and the following results were obtained. 1. The positive rate of hemoagglutination inhibit reaction (HI reaction) on swine showed 100% at maximum in the middle of September. 1980. The positive rate of 2-ME sensitive antibody showed the titer of over 1 : 40 with all swine examined in the middle of August, 1980. 2. The positive rate of HI reaction of inhabitants at Mukaiyama in Kurashiki City was 76.0% (on 8th in July, 1980) indicating the rather big decrease being compared with annual value of 90.6%, in 1976. Date indicated that increase In JE virus in swine as source of infection thought to be a vector of JE, caused a genuine case of JE, in 1980, as described below. 3. Higher positive rate of HI reaction and 2-ME sensitive positive rate of swine were observed in 1980. 4. One patient who suffered from JE was found In Kurashiki City in 1980. 5. Henceforth and continuously, Japanese Encephalitis should be investigated, above all on its occurance order with the number or Ct or the positive rate of hemoagglutination inhibit reaction in inhahitants and swine, etc. considering that J.E. has a high lethality rate and often leaves the terrible J. E. residue to patients.