A multi-beam scanner has two light emitting points ch1, ch2 and an optical system. The light emitting points ch1, ch2 are separated by a distance &Dgr;along a depth-of-focus direction. The longitudinal magnification &agr;, or the image-side numerical aperture NA, of the optical system is set so that the surface to be scanned of a photosensitive drum is positioned within a range where the depths of focus de of the light beams from the two light emitting points ch1, ch2 overlap. The multi-beam scanner can focus the light beams onto the surface of the photosensitive drum at all the scanning positions even when the optical system has curvature of field.