A beam expander means an optical component which is capable of changing a light diameter of parallel light to be adapted to an optical component at a subsequent stage. Generally, in order to configure a beam expander, it is necessary to relatively move at least two groups of lenses on an optical axis. That is, these groups are relatively moved in an optical system composed of the two groups of lenses, so as to change a combined focal length. Thereby, it is possible to make a light diameter of parallel light output from the other group different with respect to a light diameter of parallel light incident into one of the groups (for example, refer to Non-Patent Document 1).