Abstract: A light source 165 of a projection unit 16 AM modulates an electric current supplied to a built-in LED. The projection unit 16 projects the modulated light on a projection screen. A photoreceiver 171 receives, with a time lag, a reflected portion of the modulated light from the screen, which corresponds to a time required for the light to travel from its projection to its reception. A calculation unit 173 acquires a phase difference between the projected light and its reflection at a respective one of a plurality of points on the screen, acquires a distance from the projector to the respective one of the plurality of points on the screen, and then acquires an inclination angle of the screen. Because the distance from the projector to a respective one of the plurality of points on the screen is acquired based on the corresponding phase difference, the measurement accuracy is improved compared to the triangular distance measuring method.