The prediction of the weathers in coming season is important for various purposes, especially for agriculture. For the purpose of any prediction, the most conservative character is usefull for extrapolation just as equivalent potential temperature is used in the air mass analysis. Calculating the correlations between succesive monthly mean values of various meteorological elements, it is found that the air temperature is most conservative. Next, the secular change of the weather is shown for the monthly mean air temperature of Singking in January and for others and the posibility of the seasonal forecast is proved. Lastly, the prediction of the weather in winter and Bai-u season (one of the rainys eason in Japan) is investigated, but the results are not yet sufficient for the practical use and the future investigations are Roped.