The invention relates to a device for monitoring the oxygen concentration in an aircraft tank. The device includes an absorption measuring section equipped with a laser or laser diode (6), a photodiode (7), a temperature sensor (8) and a reflector (5) for carrying out laser spectroscopy on a gas volume to be measured inside the tank. The conducting components of the device are positioned outside the tank and its reflector (5) is positioned in the tank in the gas volume to be measured, the components and reflector being optically coupled by a window (3) that is situated in the tank wall (2). The absorption measuring section is mainly located in the chamber containing the gas to be measured.