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.
