PURPOSE: To simplify adjustment and correction, by giving a constitution where variance of the set time dependent upon fluctuation of the resistance value of a variable resistor is reduced.

CONSTITUTION: Charging and discharging voltage Vc of charging and discharging circuit A consisting of resistance 3, correction resistance 4, and capacitor 5 is compared with high set voltage Vd and low set voltage Vi of voltage set circuit B by comparing circuit C to generate a prescribed output. This output is applied to latch circuit D to generate output Q. Transistor 17 is turned on to discharge capacitor 5 during low-level outpt Q. Output Q is input to counter E; and when pulse input P reaches a prescribed number, output Qn changes to the high level from the low level to operate driving circuit F.