Methods of forecasting on time series data by auto-regressive model, neural
networks model and chaotic theorem are proposed. The methods are said to b
e complicated for forecasting calculations. In this paper, a simple forecas
ting algorithm by the nearest neighbor method is discussed. It is applied t
o forecasting on a monthly average temperature in Sendai city to confirm pe
rformance. The actual results indicate that the nearest neighbor method has
similar performance to the auto-regressive model in forecasting on time-se
ries data.