PURPOSE: To execute display of a zebra pattern by a simple constitution, by displaying a level of a digital signal when a prescribed host bit has coincided with a specific code, in a device which has digitized a video signal.

CONSTITUTION: Prescribed host 5 bits in a luminance signal Y among, for instance, 8 bits which is supplied to a terminal 1 are supplied to a code detecting circuit 11, and whether they coincide with a prescribed code or not is decided. In case of coincidence, its bits y3, y4 are converted to codes y3', y4' of bits decided in advance, of a digital signal, by a code converting circuit 12. Subsequently, among 8 bits of the luminance signal Y, a signal of y3', y4' are substituted for the bits y3, y4, and are outputted from a terminal 8. Accordingly, even in case of a device which is digitized a video signal, display of a zebra pattern can be executed.