A beam combining device causing beams from a plurality of light sources and one or a plurality of spare light sources to enter a beam combining optical system, and to be combined and output after passing through a beam combining element. The beam combining device is configured to: detect a failure in the plurality of light sources; and move at least a part of the respective light sources, the spare light source, and the beam combining optical system, to cause a beam to enter the beam combining optical system from the spare light source instead of a beam from the failed light source, and to cause the beam to be combined to beams from the plurality of light sources on an optical path after the beam combining element.