To perform highly sensitive gas detection with a simple structure without affecting other sensor parts even if a sensor part at an intermediate position is broken.

Pulse light locking to an absorption wavelength of gas to be measured is emitted from a light source 1 via a main optical fiber 2 to an optical means 3A1 on a first stage. Each optical means 3A separates incident light for emitting one of separated light beams via the main optical fiber 2 to an optical means 3A in a subsequent stage and emitting the other separated light beam via a branch optical fiber 4 to a gas detection sensor 5. The gas detection sensor 5 receives the separated light beam from the optical means 3A and emits it with passage light lost according to a concentration of gas existing in the vicinity. A part of the emitted light from the gas detection sensor 5 is reflected by a reflection means 6 and again incident to the gas detection sensor 5, and further lost light is incident to the optical means 3A. A detection part 7 detects concentration of gas to be measured corresponding to a position of each gas detection sensor 5 based on return light reflected via each optical means 3A.