An optical system of an optical pick-up that is capable of using optical discs DVD and CD. The system includes a light source portion for selectively emitting a first laser beam (650 nm) and a second laser beam (780 nm), and an objective lens for converging the laser beam from the light source portion onto an recording layer of the optical disc. The objective lens is provided with a transmittance controlling phase grating in a peripheral region to have a wavelength dependence such that the transmittance for the first laser beam is higher than the transmittance for the second laser beam. The transmittance controlling phase grating is formed as a plurality of concentric grooves. Each of the grooves has a rectangular cross-section in a plane containing an optical axis of the objective lens.