A combustible gas detector, which has low power consumption, low-noise and high-speed response and which enable miniaturization, a process for producing the combustible gas detector, and a fuel cell system equipped with a combustible gas detector are provided. The combustible gas detector for detecting a combustible gas includes a catalyst for reaction with the combustible gas, a first displacement unit including a flexible member, which is displaced with catalytic combustion by the reaction of the catalyst with the combustible gas, and electrical contacts, which are switched by the displacement of the flexible member in the first displacement unit.