an optical pickup apparatus and an optical recording / reproducing system including the same , the optical pickup apparatus includes at least two optical systems for different types of optical recording media , one of objective lenses of the optical systems being offset from a central line of the optical recording medium , wherein the optical system including the offset objective lens having a diffraction grating diffracting light emitted from a light source to form a main beam and sub - beams , wherein the diffraction grating includes first and second diffraction regions having different grating patterns arranged alternately thereon , and a center of each sub - beam is arranged at a boundary of the first and second diffraction regions of the diffraction grating , and a center of the diffraction grating and an optical axis of the light source are adjusted to be coincided with each other , preventing generation of an alternating current in a push - pull signal of the sub - beams .