PURPOSE: To obtain the optical system of an optical information recording/ reproducing device in which chromatic aberration is excellently compensated without increasing a number of optical elements provided in the optical system compared with a case in which the chromatic aberration is not compensated and to obtain an optical element for compensating chromatic aberration used for this optical system.

CONSTITUTION: A light beam emitted from a semiconductor laser 10 is converged on an optical disk 50 by means of an objective lens 40 and the reflected light by the optical disk 50 is separated from the optical path of the incident light beam by means of a beam splitter 30. In the optical system of the optical information recording/reproducing device, the beam splitter 30 is composed of two prisms 31, 32 joined together by a beam splitting surface 30a and a plano-concave lens 33 stuck to the prism 32 on the side of the objective lens 40 and the effect for generating chromatic aberration is given to the stuck surface of the prism 32 and the lens 33.