A light beam reflected by an information medium is split by a diffraction optical system into first diffraction light and second diffraction light, which is not a complex conjugate wave of the first diffraction light, in a predetermined energy distribution ratio. The first diffraction light enters a first photodetector (195) of a photodetector and is used for reading out data information recorded on the information medium, while the second diffraction light enters second and third photodetectors (194, 195) and is used for obtaining a servo error signal. With this optical head device, the area of the first photodetector can be reduced, thus allowing a high-speed reproduction of a data signal. Further, the first photodetector becomes less susceptible to stray light. Moreover, the number of head amplifiers, which are needed for detecting the data signal, decreases, and the S/N ratio improves.