To provide an image reduction method which holds effective pixels.

When only pixels having information of 'maximum' exist or many pixels having information of 'maximum' exist, a pixel of a maximum value out of object pixels is held (S12). When only pixels having information of 'minimum' exist or many pixels having information of 'minimum' exist, a pixel of a minimum value out of object pixels is held (S14). When only pixels having information of 'increase' exist or many pixels having information of 'increase' exist, a pixel of a maximum value out of object pixels is held (S16). When only pixels having information of 'reduction' exist or many pixels having information of 'reduction' exist, a pixel of a minimum value out of object pixels is held (S18). When a pixel of a maximum value has been held just before, a pixel of a minimum value out of object pixels is held (S20); and when a pixel of a maximum value has not been held just before, a pixel of a minimum value out of object pixels is held (S21).