PURPOSE: To maintain the phase and period of an output in a normal state even while a synchronizing signal is cut off, by measuring a difference in rise time between an output pulse from a free-running oscillator and then by controlling a ratio of division of a free-running oscillation output by the output.

CONSTITUTION: A pulse from free-running oscillator 2 drives counter circuit 14 and is divided by 1/N-divider circuit 15, whose output is made into a pulse of duty 50% by FF16 and then outputed from terminal 4. By exclusive-OR circuit 5, OR circuit 6, monostable circuit 7, and AND circuit 8, a period when an external synchronizing signal differs in rise part from an output signal from oscillator 2 is separated and this length is subtrated by up-down counter 9 when the rise of the output signal lags or added when leading, by using as a unit pulse obtained by dividing 10 the output of oscillator 2 by a number 2 or greater. As a result, when the output signal lags, circuit 15 performs division by a number smaller than usual and when leading, division is done reversely, thereby making a shift in a direction where it agrees with the external synchronizing signal.