An optical device diffracts incident light with a hologram element and receives the diffracted light with light receiving faces 20A to 29 on a light receiving element. Reflected sub-beams used for a tracking operation are received with different ones of the light receiving faces depending on the wavelengths of the reflected sub-beams. When first light receiving faces 22, 23, 26, and 27 are receiving an incident beam of a first wavelength, output signals from the first light receiving faces and output signals from the other light receiving faces 24, 25, 28, and 29 are processed to detect an unnecessary light component. The optical device can record and/or reproduce information signals to and/or from optical discs such as DVDs and CDs which need light sources of different wavelengths, without the influence of unnecessary reflected light from the optical discs or without complicating operation of output signals.