1. An image projection apparatus comprising: a projection unit adapted to project an input image onto a projection surface; an obtaining unit adapted to obtain a sensed image of a range containing an image projected onto the projection surface; a generating unit adapted to generate a difference image between the sensed image and the input image; a region detection unit adapted to detect a region corresponding to an operation object for operating the image projection apparatus, based on the difference image; a position detection unit adapted to detect information relating to a position of a particular portion of the region corresponding to the operation object detected by the region detection unit; a determination unit adapted to determine an operation carried out by the operation object, based on the information relating to the position of the particular portion detected by the position detection unit; and a control unit adapted to execute an action corresponding to the operation determined by the determination unit, wherein said region detection unit is further adapted to: perform a first detecting operation in which said region corresponding to the operation object for operating the image projection apparatus is detected based on a difference image generated by the generating unit at a first timing and a difference image generated by the generating unit at a second timing; store, in response to the detection of said region corresponding to the operation object for operating the image projection apparatus in the first detecting operation, shape feature quantity information of the detected region corresponding to the operation object for operating the image projection apparatus in a memory; and perform, based on the shape feature quantity information, a second detecting operation in which a region in the difference image generated by the generating unit, which is considered to have a high degree of similarity to a shape of the detected region corresponding to the operation object for operating the image projection apparatus, is detected as a region corresponding to the detected region corresponding to the operation object for operating the image projection apparatus.