In an image projection system, a computing device generates conversion information for converting camera coordinates used by an imaging device and projector coordinates used by an image projection device based on a positional relationship between a projection pattern image and an imaging pattern image, the imaging device images a subject including a person, the computing device detects a first position indicating a position related to the camera coordinates in a specific region in the person, the computing device converts information on the first position into information on a second position indicating a position related to the projector coordinates in the specific region based on the conversion information, and the image projection device projects a warning image for overlapping with an entire region of the specific region based on the information on the second position.