Normal region with inner wall of a lumen estimated to be normal is searched; in the normal region, a standard depth position is obtained along with the phased depth positions; a determination is made as to whether the pixel values of image data related to the phased depth positions are the pixel values of a tumor region estimated to be a tumor; when the pixel values are those of the tumor region, the deepest depth position of the tumor region is searched at each position of the inner wall of the lumen; the tumor region at each position of the inner wall of the lumen is identified for each of the deepest depth positions of the tumor region based on the tumor region searched at each position of the inner wall of the lumen and on information on the deepest depth positions of the tumor region.