Beam intensity conversion optical system of the present disclosure includes a laser light source and a beam intensity conversion lens that changes the light intensity distribution of laser light emitted from the laser light source and irradiates the light onto an irradiation region. The beam intensity conversion lens has a single-lens structure. The position of a paraxial image plane of the beam intensity conversion lens and the position of and the position of irradiation region are different from each other on an optical axis. The beam intensity conversion lens has a longitudinal spherical aberration characteristic on the paraxial image plane in which a changing range of longitudinal spherical aberration in a region of more than 70% of an image height is equal to or smaller than 20% of the amount of longitudinal spherical aberration at a position of 70% of the image height.