For the sake of reducing the number of parts and simplifying adjustment for the optical positions of the parts in an optical device wherein information signals from the light scattered or diffracted which is derived from an optical recording medium are detected, there is provided the optical device of the invention comprising a semiconductor substrate provided with a plane light emitting laser element and a photodiode; and an optical system; a layout position of the photodiode being selected in a region which extends out of the confocal region of the optical system, and from which the light scattered or diffracted from a section irradiated with laser light irradiation derived from the plane light emitting laser element with respect to the section to be irradiated.