In order to forecast the median date or the incidence rate in the prevalence of Japanese Encephalitis (JE) in Okayama Prefecture, the authors took the following factors into consideration: for deciding the median date, climatic elements, date showing immunological positivity of hemoagglutination inhibition reaction (called HI Reaction, in short) on 50 per cent number of swine and the latitude and longitude as geographical elements; and for deciding the incidence rate, the logarithm of Cule Tritaneorhychus (Ct) number and also climatic factors. They made up the multiple regression equation from the various elements in each year of 1966 to 1980 and estimated the degree of the prevalence of Japanese Entcephalitis by use of this, regression with the advancing selection method. The relation between the fact, only one patient of this disease in this Prefecture, Okayama was dIagnosed on the 10th. September, this year (1980) and the value estimated and forecased, are as followes. 1) There was not a large difference between the real date and the forecasted median date. 2) The authors forecasted and estimated the incidence rate very low and proved their forecasting to be very reliable by the fact. only one patient was dia~nosed. in the year, 1980.