PURPOSE: To make accurate color adjustment of a printed picture made by a printing device by providing a masking invertor circuit corresponding to the masking and obtaining a monitor signal from the output through a logarithmic extending circuit, a gamma-correction circuit and a clipping circuit.

CONSTITUTION: Output signal obtained by supplying output sigal of a logarithmic compression circuit 7 from a video camera 5 to a masking conversion circuit 8 is supplied to a masking invertor circuit 12. Output signal of the masking invertor circuit 12 and output signal of a logarithmic extending circuit 13 are supplied to a gamma-correction circuit 32. Outputs signal of the gamma-correction circuit 32 is supplied to a clipping circuit 33, and output signal of the clipping circuit 33 is supplied to a cathode-ray tube 34. Thus, a picture of the same tone as a printed picture printed through a color correction circuit 6 and a gamma- correction circuit 10 in a printing device P.