A scanning optical apparatus includes a light beam detection unit, a reference signal generation unit, and a returned light detection unit. The light beam detection unit decides whether the light beam reflected by the reflecting surface of a polygon mirror has entered the BD sensor, through comparison between an output value of the BD sensor and a predetermined first threshold. The reference signal generation unit generates a scan start reference signal in response to the decision that the light beam has entered the BD sensor. The returned light detection unit decides whether the light beam reflected by the reflecting surface has entered the internal light sensor, through comparison between an output value of the internal light sensor and a second threshold corresponding to light beam intensity higher than the intensity of the light beam reflected by the reflecting surface and corresponding to the first threshold.