PURPOSE: To raise the repeat reproducibility in a short time of slow start by timely cooling an NTC thermistor, in an object using the NTC thermistor having negative temperature resistance property, so as to gradually raise the number of revolutions by varying the quantity of current to a motor at power on.

CONSTITUTION: A power switch 26, a controller 22 for the hooker of the switching mechanism of a shield plate 16, an NTC 15 are connected in series, and they are connected to a motor 11 through a relay switch 24 constituted of bimetal. When a power source switch 26 is off, the heat of NTC 15 is radiated naturally by opening the shield plate 16, and also when the motor 1 gets in rated operation state, and the relay switch 24 is changed over to the NTC pass side, it is cooled compulsorily by the air of an fan by opening the shield plate 16. On the other hand, when turning on the power switch 26 to start the motor 11, the temperature of the NTC is raised by closing the shield plate 16, and the resistance value is reduced gradually to start it slowly, and when the number of revolutions goes up, it is made to perform rated revolution by changing over the relay switch 24.