PURPOSE: To realize a smooth operation control, in which protective control and overload control can be carried out fevorably, by a constitution wherein output from a power supply means is lowered upon detection of an overload current during normal operation and output from the power supply means is interrupted when overcurrent protection is required.

CONSTITUTION: When a controller 7 produces a low output L to set the output voltage from a DC power supply 8 at a low level, a motor compressor 5 is started at a low speed. Upon increase of the load current I of the motor compressor 5, a voltage appears across a detecting resistor 9. When the voltage reaches the detection level of overload control, a light emitting element 11 in the initial stage is turned ON as shown on fig. 2e and thereby a light emitting element 12 is turned ON, as shown on fig. 2c, to provide a signal (c) to the controller 7. Consequently, output from the controller 7 lowers to cause output voltage drop of the DC power supply 8 thus decreasing the load current of the motor compressor 5.