A diffraction element is positionally adjusted on a substitute optical head arrangement while an objective lens is positionally adjusted so as to maximize a total signal calculated in a photodetector, whereby predetermined focusing error signal and tracking error signal are obtained. The thus adjusted diffraction element is adhesively secured on a package housing a light source and a photodetector to form a light source unit. The thus prepared light source unit is positioned on and assembled into an optical head by angularly adjusting the light source and an objective lens on the optical head individually so that both the optical axis of the light, emitted from the light source, and incident on a recording medium and the axis of the objective lens are aligned normal to the recording medium and by coinciding the optical axis of the objective lens with the center of intensity distribution of the incident light on the objective lens. This configuration eliminates the necessity of mechanical parts on the optical head for adjusting the diffraction element and the necessity of a space for the adjustment, so that it is possible to promote the optical head small and light-weighted. In addition, by effecting the optical adjustment when the light source unit is assembled into the optical head, error signals can be adjusted. Therefore, the adjustment step can be omitted.