PURPOSE: To optimize the brightness of an image which has been inverted, and also, to vary no brightness even if γ is converted, by varying no standard gray level at the time of the inversion.

CONSTITUTION: At the time of inverting an image which has been brought to an image pickup by a camera 30, from a negative image to a positive image, or from a positive image to a negative image, it is executed so that a standard gray level is not varied at the time of the inversion. Also, in order to maintain the brightness of the whole screen so as to be constant in case the negative image and the positive image have been inverted, it is desirable especially that the standard gray level in which the brightness is not varied at the time of inversion of the negative image and the positive image is set to about 23% of a white level. In this way, the standard gray level, namely, brightness of a point of a neutral density is not varied, therefore, brightness of the whole screen becomes optimum.