PURPOSE: To obtain an optical device which has high optical effciency with a small number of component parts of an optical system and the facilitated control of the optical system, by unifying a lens which leads the light beam transmitted from a semiconductor laser onto a disk and a prism which extracts the reflected light to form one body.

CONSTITUTION: The light beams transmitted from a semiconductor laser 1 become approximately parallel to each other through prisms 2 and 3 and the 1st lens 5 and are irradiated on a disk 8 in the form of a minute spot through the 2nd lens 6. The reflected beams are reflected again on the junction surface A of the prisms 2 and 3 and received by a photodetector 9 through a lens 4. The flat surface 5a of the lens 5 is cemented to a surface 3a of the prism 3 in a monolithic structure. This eliminates the vapor deposition of a reflection preventing film on the surface 3a (5a), and an optical loss is reduced. Furthermore the control is facilitated for an optical system.