PURPOSE: To obtain an insulation detector which has a reduced output voltage detecting ripple and a high response by switching sample and hold of an auxiliary winding output voltage of the detector in response to ON, OFF of a main switch element.

CONSTITUTION: The input side of a photocoupler 10 is connected to the secondary winding 1b of a transformer 1 through a Zener diode 9. The output side of the photocoupler 10 turns ON, OFF a switching transistor 2 through a transistor 11 to control a secondary side DC voltage to a desired value. An FET 29 as an auxiliary switch element is connected to one terminal of a third auxiliary winding 1e of the transformer 1. If an output voltage V1 does not reach a Zener voltage VZD, the FET 29 is turned OFF to hold the value of the voltage V3 of the winding 1e before entering an intermittent period. If the voltage V1 exceeds the voltage VZD, the FET 29 is turned ON to sample the voltage V3 as it is.