The authors developed a simple model to estimate the long term transition of soil moisture tension (daily average pF value at 15cm depth in soil) for the next day (PFi+1) by the following formula, where PFi, Ei, Pi and PFmin are, pF value, daily total amount of evapotranspiration (mm·day-1), daily amount of precipitation (mm·day-1) and the minimum PF value of the field after heavy rain fall, respectively: PFi+1=α(Ei-Pi)+β·PFi+γ (when calculated PFi+1 is larger than PFmin); PFi+1=PFmin (when PFi+1 calculated by above equation is less than or equal to PFmin). The constants α, β and γ for the beet field in 1993 were 0.025a, 0.876 nd 0.25, respectively. PFmin was 1.9 for three crop fields in 1993. The standard estimate errors in PF by the equations were less than 0.2 for three crop fields in 1993.