A scanning optical apparatus according to the present invention comprises a light source (2), a modulator (1) for modulating a light flux emitted from the light source (2), an image forming device (6a, 6b, 6c) for forming an image of the light flux from the light source to a predetermined position, a deflector (5) for deflecting the light flux from the light source in a predetermined position, a photosensitive device (8) for sensing the light flux deflected by the deflector (5), an adjusting device (10) for adjusting an image forming position of the light flux by the image forming device (6a, 6b, 6c) in correspondence to a signal obtained by the photosensitive device (8) and an inhibiting device (11) for inhibiting the operation of the adjusting device (10) when the light flux emitted from the light source (2) is being modulated by the modulator (1) in correspondence to a predetermined signal. By the scanning optical apparatus of the invention, the focal deviation can be always stably corrected and a stable image with high density can be obtained.