1. An image projection system comprising: a projection portion to project a projected image onto a projection surface; a drawn-image detecting portion to detect a drawn image drawn on said projection surface while said projected image is being projected by said projection portion; and a modification portion, in the case where said drawn image is detected, to specify from said projected image which is projected onto said projection surface a part including at least a drawn image part overlapping said detected drawn image, and modify the specified part of said projected image, on the basis of the detected drawn image, so as to emphasize or cancel said drawn image.