A new method (HUMIDAS) for estimating relative humidity from data of AMeDAS (Automated Meteorological Data Aquisition System) was developed by multiple regression analysis. In this method, relative humidity (Y) for a 3-hr period is expressed as follows: Y=AX₁+BX₂+CX₃+DX₄+EX₅+F. X₁=temperature, X₂=precipitation, X₃=wind velocity, X₄=shining hour and X₅=range of temperature. The A-E values are regression coefficients and F is a constant. The relative humidity actually observed agreed well with the estimated values.