Optical system includes front group, light-shielding member, and rear group that are arranged from object side toward image side. The light-shielding member is provided with opening elongated in first direction. The front group has aspherical surface, does not image the object at the opening in first section parallel to the first direction, and forms intermediate image of the object at the opening in second section perpendicular to the first direction. The rear group has diffractive surface that splits light beam that passes through the opening into light beams at different wavelengths in the second section and focuses the light beams on different locations in the second section. The curvature radius of the aspherical surface in the second section at on-axis position in the first direction differs from that at outermost off-axis position in the first direction.