PURPOSE: To measure the time elapsed before a thermal switch is opened after a power source was closed with good accuracy, by mounting a first switch for controlling the flow of the preheating current of the thermal switch, first and second DC power sources and a voltage monitor circuit.

CONSTITUTION: A first switch 45 is connected to the thermal switch 11 of a starter A in series and controls the flow of the preheating current of the thermal switch 11. A first DC power source 43 is connected to both terminals of the series circuit of the switch 45 and the switch 11 through a preheating current adjusting resistor 46 and a second DC power source 44 is connected to a main heater 15 through a second switch 48 for controlling heating to apply voltage. A voltage monitor circuit 47 detects the voltages at both terminals of the series circuit of the switch 11 and the switch 15 changing by the opening and closing of the switch 45 and the switch 48 is closed when the voltage of the series circuit is a low level and opened when a high level. Then, a measuring circuit measures the time elapsed before the switch 11 is opened after the switch 45 is closed. By this method, pulsation is eliminated and the measuring of resolving power of an operation time is enhanced.