Patent ID: 8487974

Claim:
An image forming apparatus comprising: first, second, third, and fourth light source units configured to emit first, second, third, and fourth light beams, respectively; a deflecting unit having a first deflecting surface and a second deflecting surface that is different from the first deflecting surface, the first deflecting surface being configured to deflect the first and second light beams for scanning and the second deflecting surface being configured to deflect the third and fourth light beams for scanning; a first imaging optical system configured to guide the first and second light beams deflected by the deflecting unit onto first and second surfaces to be scanned, respectively; a second imaging optical system configured to guide the third and fourth light beams deflected by the deflecting unit onto third and fourth surfaces to be scanned, respectively; and a beam detecting optical system disposed at a position nearer the second imaging optical system than the first imaging optical system, wherein a third optical path length from the third surface to be scanned to the second deflecting surface and a fourth optical path length from the fourth surface to be scanned to the second deflecting surface are longer than a first optical path length from the first surface to be scanned to the first deflecting surface and a second optical path length from the second surface to be scanned to the first deflecting surface.