In a laser scanning optical system, at least one of one or more scanning optical elements is made of a material having a photoelastic coefficient equal to or greater than 20×10−12 [Pa−1]; wherein at least one of one or more reflectors comprises a basal plate, and a metal film and a single-layer optical thin film evaporated on the basal plate; wherein the single-layer optical thin film has a thickness greater than 0.15λ and less than 0.40λ, wherein λ is a wavelength of the light beam; and wherein the light beams heading to both ends of an effective scanning range of the imaging surface, of which image heights are maximum, enter the one or more reflectors at angles of incidence equal to or greater than 10 degrees and less than 55 degrees.