PURPOSE: To enable ready adjustment of a time from the entrance and vanishment of an AC voltage to the generation and vanishment of an output signal in a high frequency voltage detecting circuit by providing a transistor conducting and shutting off by the voltage at a smoothing capacitor.

CONSTITUTION: First smoothing circuit is formed by resistors R1, R2 and a capacitor C, and second smoothing circuit is formed by resistors R3, R4 and a capacitor C1. The capacitor C1 is connected to a transistor (Tr) 12 at its base to control the operation of the transistor 12 by the voltage at the capacitor C1. When an AC voltage is applied to a termianl 1, the capacitor C1 is charged to thereby cut off the transistor 12, and the capacitor C is charged at the time constant of τ1'=R1C. When the AC voltage becomes zero volt, the transistor 12 is conducted to cause the capacitor C to dischrage at the time constant of τ2'=R2C(R1R2). Since the delay times τf, τr are set by the time constants τ1', τ2', it can be readily set τf>τr in case of R1R2.