To provide a compact photoelectric fluid sensor of high sensitivity capable of easily managing a position of a photodetector.

Two sheets of reflection mirrors 13a, 13b are mounted in such a state that their reflecting faces as recessed faces are opposite to each other, and a gas is introduced between the reflecting faces of the reflection mirrors 13a, 13b. This sensor comprises a light source 21 radiating the light to the gas, and the photodetector 22 receiving the light radiated from the light source 21 and reflected by the reflection mirrors 13a, 13b. Dimensions and positions of the reflection mirrors 13a, 13b, the light source 21 and the photodetector 22 are specified to allow the light radiated from the light source 21 to be incident on the photodetector 22 as a parallel beam of light in a first plane including an optical axis of the photodetector 22.