PURPOSE: To prevent a frequency discriminator from being malfunctioned because of discontinuous signals and dropout, by making a constant current source inoperative when a count value of a counter circuit is brought only once to a state at the outside of a prescribed range.

CONSTITUTION: T-FFs 25, 26, 27, frequency-divide a horizontal synchronizing signal separated from a video signal by 1/m, where (m) is a positive integer. Further, a counter 24 is controlled with an output of a frequency division circuit and counts a pulse inputted at an nH period, where (n) is a positive integer. When the count value is more or less for a prescribed value than a reference numeral for ≥2 consecutive values, the counter 24 drives a constant power supply 28 or 29 so as to output an output current of opposite polarity.