A video processor receives first and second images respectively acquired by an endoscope at first and second timings before and after a predetermined function is applied to a subject, generates first and second correction images respectively using first and second images, generates first and second post-correction images respectively obtained by causing the first and second correction images to act on the first and second images, extracts color components in the second post-correction image to find first and second distribution characteristic values, and calculates a degree of change of the second distribution characteristic value from the first distribution characteristic value.
