SCANNING OPTICAL DEVICE

To provide a scanning optical device having a coupling lens with a diffraction surface in which image forming state hardly changes due to temperature variation.SOLUTION: A scanning optical device 10 comprises: a semiconductor laser 1; a coupling lens 2 converting light from the semiconductor laser 1 into a beam; an optical deflector 50 deflecting the beam from the coupling lens 2; a scanning optical system 40 imaging the beam deflected by the optical deflector 50 on a scanned surface 9A; and a holder HL holding the semiconductor laser 1 and the coupling lens 2. The coupling lens 2 includes a refractive surface with positive refractive power and a diffraction surface with positive diffraction power. A ratio φn/φd of refractive power φn of the refractive surface in a main scanning direction to diffraction power φd of the diffraction surface satisfies 1.85≤φn/φd