PURPOSE: To prevent damage of a power-supply unit due to excess of voltage- application time and at the same time to prevent application of an inverse voltage before starting up the power supply due to a measurement procedure mistake in a device which performs a test of an output excessive voltage by applying the inverse voltage to an output terminal of the power-supply unit.

CONSTITUTION: When a switch 9 is turned on, a one-shot circuit 2 is triggered through a chattering-prevention circuit 1. A time-delay circuit 3 outputs a signal only for a certain amount of time by the one-shot circuit 2, thus enabling a transistor Q3 to be conducted, photo transistors PC1-Q1 and PC1-Q2 to be turned on and then an output voltage of a DC power supply 11 to be connected to an output terminal to be tested of a power-supply unit 5. In this manner, the maximum and minimum values within a range of excessive voltage detection can be performed.