To perform irrigation indispensable to the cultivation/raising of plants under inference of the water content of soil by using the value of electric resistance of impurities in soil and/or water.

The starting time and the stopping time are set on the basis of the water content of soil by inferring the water content through the measurement of values of electric resistance of impurities contained in soil before and after irrigation. The values of electric resistance of soil are sensed with two sets of measuring terminals (R1K1) and (R2K2) and a thermistor (TH) for responding to temperature change, the above times are set with both of a water supply starting switch (S2) and a water supply stopping switch (S3), and a motor and a pump are actuated with a control circuit to perform irrigation.