1. An image processing apparatus comprising: a CPU and memory, programmed to operate as: a designation acceptance unit configured to accept designation of a first diseased part region from a first image of an object to be examined by a medical doctor; a determination unit configured to determine a first diseased part reference region, of the object to be examined by a medical doctor, surrounding the diseased part region accepted by said designation acceptance unit; a feature amount extraction unit configured to extract a feature amount in the diseased part reference region, of the object to be examined by a medical doctor, determined by said determination unit; a registration unit configured to register the feature amount in the diseased part reference region extracted by said feature amount extraction unit into dictionary data; a detection unit configured to detect from a second image a corresponding region corresponding to the diseased part reference region using the dictionary data registered by said registration unit; and a determining unit configured to determine whether a second diseased part region is included in the second image based on whether the corresponding region detected by said detection unit is included in the second image.