A projection display system includes: an image projection device projecting an image on a projection surface; an optical detecting device; and an attaching device, wherein the optical detecting device includes an irradiating section emitting an irradiation light toward a detection region which is set along the projection surface, a light receiving section receiving a reflected light generated as a result of the irradiation light being reflected from the object, and a detecting section detecting position information of the object based on the result of the light received by the light receiving section, and the attaching device includes a first attaching section attached to a support supporting the attaching device, a second attaching section attached to the image projection device, and a third attaching section attached to the optical detecting device.