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. The light receiver is disposed at the condensing position by the concave mirror within the housing and outputs an electric signal for detecting whether the gas to be detected exists or not at a front portion of the material body, in accordance with the degree of damping of the light component having the wavelength corresponding to the absorption spectrum of the gas to be detected, by receiving the laser beam which returns via the light transmitting portion. IMAGE
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