A light source has a tunable wavelength single mode semiconductor laser. The laser outputs a single mode laser beam having a wavelength tuned in accordance with a drive current and emits, for an object to be detected, at least a first laser beam having a first wavelength which is absorbed by a gas to be detected and a second laser beam having a second wavelength which is not absorbed by the gas to be detected. A controller switches the drive current having a predetermined value corresponding to the first or second wavelength at a predetermined period and supplies the drive current to the laser and controls the drive current having the predetermined value corresponding to the first or second wavelengths so that the first or second laser beams are alternately output from the laser with substantially the same power while the first and second wavelengths are maintained. A light-receiving unit receives first and second response light components generated when the first and second laser beams emitted alternately from the laser are incident on the object to be detected and outputs electrical signals corresponding to received light amounts of the first and second response light components. A signal processor receives the electrical signals from the light-receiving unit and processes the presence/absence of gas detection in accordance with a difference between the received light amounts of the first and second response light components.
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