Abstract: The present invention provides a laser absorption spectroscopy type gas detecting device having a light source device, a light receiver, and a concave mirror which are disposed within a housing, and aiming to be compact and lightweight. The light source device emits a laser beam, containing a light component having a wavelength corresponding to an absorption spectrum of gas to be detected, to the exterior via a light transmitting portion. The concave mirror has a reflecting surface making the laser beam, emitted to the exterior by the light source device and reflected by a material body existing in an advancing direction of the laser beam and having a property of reflecting light and returning it via the light transmitting portion, be reflected and condensed at a predetermined condensing position.