An image projection system includes a projector and a light pen. The projector includes: an image pickup unit which performs image pickup in a periodically repeated image pickup period and does not perform image pickup in a non-image pickup period; a detection unit which detects a pointed position of the light pen; and an infrared signal receiving unit which receives pointing element information transmitted from the light pen. The light pen includes: a light emitting unit; a control unit; and a storage unit which stores the pointing element information. The light pen performs first light emission in the image pickup period and second light emission corresponding to the pointing element information in the non-image pickup period. The projector detects the pointed position, based on an image of the first light emission picked up by the image pickup unit and receives the pointing element information transmitted via the second light emission.