PURPOSE: To execute signal detection only by one judgement and to reduce a detection time by inhibiting the count of signals in a vertical synchronizing period and an equal pulse period at the time of judging the existence of a main input signal to prevent the misdetection of a signal as noise.

CONSTITUTION: When a switch circuit SW3 selects the 1st input, a microcomputer 10 supplies the input 1 to a video processing circuit/synchronizing separator circuit 8 through a selector circuit 6 and a switch SW1. The circuit 8 supplies signals to the microcomputer 10 during vertical synchronizing and equal pulse periods. A synchronizing separator circuit consisting of transistors(TRs) Q1, Q2 obtains vertical and horizontal synchronizing pulses from the input 1 selected by a switch circuit SW3 and the microcomputer 10 counts the pulses to judge the existnce of a VHF/UHF input signal. When 39 to 41 pulses can be counted up based upon 40 pulses e.g. in the counting operation of the microcomputer 10, the existence of a VHF/UHF input signal is judged. thereby a pulse having a 2fH period can be detected only by one counting operation without being misdecided as noise.