PURPOSE: To realize the simultaneous viewing of the images of three waveforms R, G and B for level display, by mixing plural waveform video signals into the video signal of a viewfinder with switching in terms of time.

CONSTITUTION: The switches S1, S2 and S3 are switched successively by the pulse signal. Thus the output of the emitter follower of a transistor Q4 arranges the signals R, G and B in a time series form which is switched with every horizontal scan or vertical scan. The video signal viewed through a viewfinder VF is fed to the VF through a terminal 4, a transistor Q5, a resistance R7 and a terminal. In this case, the time series signals R, G and B are mixed into the videosignal of the finder VF via a resistance R5. Thus the images of three waveforms R, G and B for level display which are superposed on the image can be viewed at one time through the finder VF.