PURPOSE: To make a gamma characteristic or the like being able to be revised optionally by employing digital signal processing for the unit.

CONSTITUTION: Only a difference of input and output signals before and after prescribed nonlinear processing is stored in a ROM 20A and a relevant difference is read from the ROM 20A based on an inputted digital signal. An optional correction factor γ is multiplied with the read difference by a multiplier 20C and the product is added to an inputted digital signal by an adder 20B. Thus, a correction quantity (that is, a value for nonlinearization such as gamma correction) is revised freely by selecting properly the correction factor γ to be multiplied with the difference by the multiplier 20C.