To provide a miniaturizable combustible gas detector having a small power consumption, a low noise and a high-speed response, a manufacturing method of the combustible gas detector, and a fuel cell system loaded with the combustible gas detector.

The combustible gas detector for detecting a combustible gas has a configuration having a catalyst 1 to be reacted with the combustible gas, the first displacement means 2 constituted of a flexible body to be displaced following catalytic combustion by a reaction with the combustible gas of the catalyst, and electric contacts 3, 4 to be switched by displacement of the flexible body in the first displacement means.