When charges Q (x, y) transferred from an image inputting device 1 are to be converted into first signal intensities S' (x, y) and signal processing is to be performed for the first signal intensity S' (x, y) of a particular pixel, a maximum value Smax, a minimum value Smin and an average value Save are calculated from signal intensities S' (x-1, y) and S' (x+1, y) at adjacent pixels. When S' (x, y)>Smax.times.A is satisfied, it is determined that the signal intensity S (x, y) at the particular pixel=Save.times.C (where A and C are coefficients), whereas when S' (x, y)