PURPOSE: To contrive to reduce effect of noise in the latter stage treatment by checking noise in the Poisson distribution by a non-linear square root conversion or the like of an input pixel signal.

CONSTITUTION: An input pixel signal A/D converted is subjected to a non-linear conversion such as square root conversion with a square root conversion table 4 and added to the contents of an image memory for storing the signal with an adder 5. Noise components are checked in the Poisson distribution of input pixel signals with an average addition determined by repetition of this operation. The contents of the memory 7 are processed with an equalization circuit 8, a square conversion table 9, a normalization circuit 10 and the like and an output is produced through a selector 6, an output unit 11 and the like. This can reduce effect of noise in the latter stage treatment of the pixel signals to form an image better in the concentration, profile or the like.