PURPOSE: To prevent rise in output voltage regardless of forgotten connection of a remote sensing line by applying a voltage other than the DC power cource output higher than detected value of any overvoltage to the input of an overvoltage detection circuit through a resistor.

CONSTITUTION: A voltage higher than a detection value of overvoltage is applied to an overvoltage detection circuit 9 through a resistor 11 and a diode 12 from a constant voltage power source 10 beforehand. Here, a voltage of the constant voltage power source 10 and a resistor are set appropriately. As a result, when a remote sensing line is connected to a load terminal 5, an input voltage of an error detection circuit 8 and the overvoltage detection circuit 9 is made to depend on the load terminal 5. When the remote sensing line 7 is disconnected from the load terminal 5, a voltage of the constant voltage power source 10 is applied through the resistor 11 and detected as overvoltage with the overvoltage detection circuit 9 which outputs an overvoltage detection signal 13 to halt the power supply. With such an arrangement, rise in the output voltage can be prevented even when forgotten connection or the like of the remove sensing line takes place.