An optical pickup compatible with two wavelengths such as an optical pickup having a two-wavelength, multi-laser source unit to be compatible with both DVDs and CDs prevents, in a simply structured optical system, error signal quality deterioration caused by relative shifting between laser beams and realizes detection of focus and tracking error signals. The optical system is provided with an optical waveguide device and a photodetector. The optical waveguide device includes a holographic diffraction grating having three regions divided by two approximately parallel division lines which extend respectively passing the optical axes of a laser beam for DVD and a laser beam for CD. The photodetector has plural light receiving surfaces each divided into three regions. This allows reliable focus and tracking error signals to be detected during both DVD reproduction and CD reproduction.