To detect the position of an image sensor with high accuracy without using a special position detection device such as a hall element.

First, a boundary between a portion on the image sensing surface of an image sensor on which light is irradiated by image formation and a dark portion thereon on which light is not irradiated is projected in a direction diagonally displaced relative to the row or the column direction of a pixel array. Next, by a position detection unit, a pixel value is retrieved from plural pixels onto which the boundary is projected, and a brightness distribution in the plural pixels is calculated. Next, by the position detection unit, a change point between brightness where the area on which pixel light is irradiated is half or more in the brightness distribution and darkness where the area on which pixel light is irradiated is less than half is detected. Then, by the position detection unit, the position of the image sensor is calculated from the position of the change point between brightness and darkness.