A beam-combining laser beam source device comprising an airtight housing and a laser beam source unit housed in the housing. The laser beam source unit comprises laser beam sources and collimator optical systems respectively positioned in optical paths of laser beams, which are radiated from the laser beam sources, in order to collimate the laser beams. Optical path adjusting elements are respectively positioned in the optical paths of the laser beams in order to radiate the laser beams along optical paths parallel and close to one another. The laser beam sources, the collimator optical systems, and the optical path adjusting elements are supported on a single support. The housing is provided with a temperature sensor, which detects the temperature in the housing, and temperature adjusting elements which heat or chill the housing on the basis of control of the temperature sensor so that the temperature in the housing is kept constant.