PURPOSE: To constitute a PLL circuit without using a capacitor, a crystal and a voltage controlled oscillator, etc.

CONSTITUTION: The period of a regenerated horizontal synchronizing signal inputted by a period counter 12 is measured and a mean value arithmetic operation circuit 14 calculates the means value and stores the result to a mean period register 15. A window control circuit 17 monitors a mean period stored in the means period register 15 to decide whether or not the period is within the range of a prescribed window. A window control circuit 18 controls the period of an FH pulse outputted by an FH counter 21 corresponding to the result of decision of the window control circuit 17.