At least one plane diffraction grating is formed on a rotatable plate, and a light beam is made to impinge upon the plane diffraction grating. The first-order diffracted light component and the minus first-order diffracted light component of the light beam diffracted by the plane diffraction grating are reflected by use of many plane reflection surfaces and guided again to the plane diffraction grating so that, after the reflected light beams are diffracted again by the plane diffraction grating, the light beams diffracted again become parallel with the original light beam incident on the plane diffraction grating. The parallel light beams diffracted again are focused by a focusing lens onto a scanning surface, so that the scanning surface is scanned with the focused light spot along a bow-free straight scan line as the rotatable plate is rotated.