1. An image processing apparatus for performing an image processing on a body cavity image captured in a living body, comprising: a storage unit which stores information including image information of the body cavity image; a change amount calculator which reads out the image information of the body cavity image from the storage unit and calculates, in the read body cavity image, a pixel value change amount of a pixel of interest with a plurality of surrounding pixels located around the pixel of interest; a candidate lesion region detector which detects a candidate lesion region in the body cavity image based on a calculation result by the change amount calculator; an inter-surrounding-pixel change amount calculator which calculates a surrounding pixel value change amount between the plurality of surrounding pixels; and a non-target pixel determining unit which determines a non-target pixel to be excluded as one of a calculation target by the change amount calculator and a calculation target by the inter-surrounding-pixel change amount calculator.