An aberration corrector and a method to reducing a spherical aberration are disclosed. The aberration corrector has a radial, rotationally symmetric variation of refractive index including a term varying in proportion to a fourth degree of a distance from the optical axis. Since the spherical aberration causes a wavefront deviation proportional to the fourth degree of distance from the optical axis, the spherical aberration can be reduced by the aberration corrector when its thickness causes the exact amount of the phase delay corresponding to the wavefront deviation, but with an opposite sign.