To prevent malfunctions by detecting the duty of the reference pulse of an f-v voltage at the time of temperature change, so as to correct the duty of a reference pulse of the f-x voltage.

A frequency of a signal which is to be basic is stored in a storing part 2, while adjusting the duty of a basic pulse by a resistance 6 (R1) through a multivibrator 5. A frequency-discriminating signal (b) is fetched through a frequency-discriminating part 1 to be inputted to the part 2, and data (c) of a previously stored pulse and a pulse at a present time point are added to a pulse-comparing part 3, according to the signal (b) to execute comparing operation. Its difference data (d) are added to a reference voltage control part 4 to control the reference voltage of a mono-stable multivibrator 5 to control the duty of a pulse to be outputted. Consequently, the duty of the pulse to be outputted from the multivibrator 5, which becomes the reference pulse of an f-v voltage by temperature change is constant at the same frequency. The f-v voltage is fetched through an integration circuit, constituted of a resistor 7 (R2) and a capacitor 8 (C2).