To provide a beam shaping optical system which is hardly affected by a temperature change.

A collimating lens 403 is held in a cell 404 and is installed at the bottom of a housing 401 by aligning its optical axis to a light source 402. A glass material which is negative in a refractive index temperature change dn/dt is used for the glass material of the collimating lens 403, by which the component of the focal length of the collimating lens 403 changed by thermal expansion is canceled by the change in the refractive index and the quality of the collimating light not dependent upon the temperature change is obtained. The beam shaping optical system which is tolerant to the temperature change and is highly reliable can be thereby realized. When an LD is used for the light source 402, a light emission wavelength fluctuates by accompanying the temperature change and the change in the focal length of the collimating lens 403 is canceled by the refractive index change of the lens glass material by this wavelength fluctuation and the refractive index change of the collimating lens glass material by the temperature, by which the quality of the collimating light not dependent upon the temperature change can be obtained.