An image processing apparatus for endoscope includes a processor. The processor sequentially receives a plurality of observation images of an endoscope, detects one or more lesioned parts from each of the plurality of observation images, analyzes visibility of at least one of a distance from the endoscope, an occupied area, a shape, a size, a position in the observation image, a color, luminance, or an organ part relating to the detected lesioned part, sets a display extension time of a detection result of the lesioned part according to the visibility, and outputs the observation image to which the detection result of the lesioned part is added.