PURPOSE: To reduce the level dispersion of a synchronizing signal portion.

CONSTITUTION: An output of a SYNC tip clamp circuit 2 connects to a 1st input of a switch changeover control signal generating circuit 3 and an electronic switch circuit 5. A DC voltage source 4 with an equal characteristic as the SYNC tip clamp circuit 2 is connected to a 2nd input of the electronic switch circuit 5 and output of the switch changeover control signal generating circuit 3 is connected to a control terminal of the electronic switch circuit 5. Since the switch changeover control signal generating circuit 3 is a circuit to generate a control signal having the same timing as that of the synchronizing signal portion of an input signal, the electronic switch circuit 5 output a usual input signal as it is and the input signal is converted into a DC voltage of the DC voltage source 4 for the timing of the synchronizing signal only and outputted. Thus, a flaw or noise of a waveform in the synchronizing signal portion is eliminated and the dispersion in the levels of the synchronizing signal portion is reduced.