To provide a photoacoustic gas sensor suitable for measuring the concentration of different gases containing an explosive gas.

A tubular sensor body 1 comprises a tubular measuring cell 13 having longitudinal axis extending vertically to the longitudinal axis of tubular sensor body 1. Consequently, the length of measuring cell 13 can be varied depending on a desired sensitivity and range of measurement without varying the outside dimensions of sensor body 1. A light source 5 is disposed such that the measuring cell 13 is irradiated with light but a film 2 is not irradiated and thereby no disturbance signal is generated. The gas sensor is made explosion-proof by enclosing the light source 5 and a photodiode for monitoring the light source 5 tightly from the environment. Function of a microphone, light source 5 and photodiode is monitored by analyzing a background signal.