An edge detecting device for detecting an edge of a moving recording medium includes a reflecting unit that includes a reflecting surface having reflectance higher than that of the recording medium, light sources that irradiate the recording medium and the reflecting surface of the reflecting unit with light, a light receiving unit that includes pixels of a photoelectric conversion element provided in a width direction of the recording medium and receives reflected light that has been emitted from the light sources and has been reflected by the recording medium and the reflecting surface of the reflecting unit on the pixels of the photoelectric conversion element, and a control unit that determines a region of the pixels of the photoelectric conversion element that make output higher than a previously set determination threshold value to be the edge of the recording medium on the light receiving unit.