An optical pickup which appropriately detects reflected light and return light reflected back from an information recording medium even if a laser light B1 emitted from a light source varies in wavelength. The transmittances of a half mirror to the laser light B1 emitted from the light source, and to the reflected light Bwr, Ber or the return light Brr reflected back from the information recording medium are preset according to a predetermined condition, i.e., Tpc(.lamda.c)/Tp(.lamda.)=(1-Ts(.lamda.)/(1-Tsc(.lamda.c)).times.(Tsc(.la- mda.c)/Ts(.lamda.)). Here, .lamda. is a predetermined wavelength emitted from a semiconductor laser as the light source, Ts(.lamda.) is a transmittance to the laser light B1 with a predetermined polarization, Tp(.lamda.) is a transmittance to the reflected light Bwr, Ber or the return light Brr reflected back from the information recording medium when the laser light B1 with the predetermined wavelength .lamda. is emitted, Tsc(.lamda.c) is a transmittance to the laser light B1 with a wavelength .lamda.c when the semiconductor laser varies in wavelength characteristic, and Tpc(.lamda.c) is a transmittance to the reflected light Bwr, Ber or the return light Brr reflected back from the information recording medium when the laser light B1 with the wavelength .lamda.c is emitted.