An aberration correcting device is placed in an optical path extending from a light source of an optical pickup apparatus for irradiating a light beam onto an optical disk to the optical disk. The device includes a pair of light-transmission substrates at least one of which is held freely movable in a way that a major surface thereof moves in a direction vertical to the optical path, and which have complementary curved surfaces facing, and apart from, each other. The curved surfaces are formed in such a manner as to cause the optical path length of a light beam transmitting the pair of light-transmission substrates to be changed by movement of the light-transmission substrates, thus imparting a phase difference to a transmitting light beam, while allowing the phase difference to keep an advancing direction of the light beam thereby to minimize a comatic aberration caused by the optical disk.