PURPOSE: To prevent the damage of the side of a load due to an excessive voltage, by providing a smoothing circuit with a fuse and a constantvoltage diode at the outputting side of the same.

CONSTITUTION: A fuse 9 is designed so as to be fused by an allowable current due to the fluctuating rate of a load 8 while the dielectric strength of a constant-voltage diode 10 is designed at a value whereat an output voltage V0 is shifted to a short- circuit condition when it has arrived at the value of the allowable voltage of the load 8. Accordingly, when an abnormality is generated in the load 8 and a current, higher than the allowable current, is conducted, the fuse 9 is fused and circuits 6, 4, 3 are protected. On the other hand, when a short-circuit accident is generated in the transistor 4a of a switching circuit 4, the high voltage V1 of a rectifying circuit 3 is not processed by chopping and is inputted into a smoothing circuit 6 whereby a high voltage V1, exceeding a specified voltage V0, is outputted between output terminals 7a, 7b. This voltage exceeds the allowable voltage of the load 8, therefore, the dielectric breakdown of the constant-voltage diode 10 is caused and a shortcircuit current is conducted through the constant-voltage diode 10 whereby the fuse 9 is fused. Accordingly, the impression of the excess voltage on the road 8 may be prevented.