PURPOSE: To reduce shortcircuit current by providing a second conduction ratio suppression circuit for suppressing the conduction ratio of intermittent input current when the voltage detected through a voltage detecting means is lower than a predetermined lower limit.

CONSTITUTION: When the current (i) flowing through an input side circuit 2 exceeds over a rated current (in) and reaches to the overcurrent set value (io) of a first conduction ratio suppression circuit 20, the first conduction ratio suppression circuit 20 starts to function with protection characteristic (c) where the voltage V drops abruptly but the current (i) continues to increase slightly. When the voltage V drops to the lower limit (vm) according to the characteristic C, a second conduction ratio suppression circuit 40 starts to function. protective characteristic A1 is achieved for maximum suppression of the second conduction ratio suppression circuit 40 and protective characteristic A2 is achieved for minimum suppression of the second conduction ratio suppression circuit 40. By such arrangement, shortcircuit current can be reduced below the overcurrent set value.