A reproducing/recording optical pickup compatible with discs having different thicknesses includes a transparent plate having a central portion formed with a diffraction grating pattern for diffracting the light travelling from the light source to the objective lens, and a transparent portion through which the light passes, and a photodetector having first and second light receiving portions. The zero-order transmitted light and the passing light are focused using all regions of the aperture of the objective lens with respect to a thin disc. The positive first-order diffracted light diffracted by the diffraction grating pattern of the transparent plate is focused onto a thick disc by partial regions of the objective lens. Then, the reflected lights thereof are detected by the first and second light receiving portions, respectively. Because signals for reproduction and servo for each disc are detected as the detection signals of the first and second light receiving portions, stable servo and noiseless reproduction are possible, irrespective of the thickness of the disc used.