There is disclosed an optical device in which a first light source for outputting a first wavelength light is apart from a second light source for outputting a second wavelength light by a predetermined distance. An information recording medium is irradiated with the first and second wavelength lights transmitted through a holographic optical element having first and second diffraction areas. The first and second diffraction areas are provided with grating arrangements in which grating axis directions are parallel to each other and grating pitches are different from each other. The first and second wavelength lights reflected by the information recording medium are transmitted through the holographic optical element and diffracted by the first and second diffraction areas. The first and second wavelength diffracted lights by the first diffraction area are converged to much the same first position on a light receiving element substrate, and the first and second wavelength diffracted lights by the second diffraction area are converged to substantially the same second position on the light receiving element substrate. First and second light receiving elements are disposed in the first and second positions, respectively.