At least one of a coupling lens and a cylindrical lens has a diffraction lens surface having a ring-band structure with a height h between adjacent ring-bands. The diffraction lens surface has an area A having a shape similar to that of the ring-band structure with a height h′, where h≠h′. The ring-band structure has a function of correcting a fluctuation in focal position of the scanning beam on the scanning surface, and the area A has a function of expanding a focal depth of a light spot on the scanning surface.