PURPOSE: To prevent the consumption of contacts and the generation ot a poor adhesion by measuring the variation of resistance value of a magnet relay coil to predict the temperature scope of the coil, and controlling the driving timing of the magnet relay with the predicted value.

CONSTITUTION: The resistance value of a coil 3 increases about 12% as the coil temperature rises 30°C. Therefore, by making the resistance value of the coil 3 when the coil temperature is 30°C as r30Ω, and the resistance value of the resistance R1 as r1Ω, the resistance r2 of the standard resistance R2 is selected as r1+r30Ω, and the resistance r3 of the standard resistance R3 is selected as r1+1.12.r30Ω. As a result, by comparing the potentials fed through the resistances R1 to R3 in a comparator 6, the variation of resistance value of the coil 3 can be detected. In a zero cross control circuit 7, the operation time to the coil temperature is stored beforehand, and a zero cross control signal 13 of the relay operation timing is set and output by the coil resistance data 11 from the comparator 6 and the zero cross phase signal 12 of an AC power source 4. A driving circuit 8 receives the signal 13 and outputs a coil driving signal 15.