NOVELTY - A fuel cell system consists of fuel cell, system stop controller (30), and hydrogen-concentration reduction unit. The fuel cell has fuel electrode, oxidizing-agent electrode, and electrolytic membrane. The system stop controller stops driving of the system in a state in which hydrogen exists in the oxidizing-agent electrode. The reducing unit reduces the hydrogen concentration in a downstream from the oxidizing-agent electrode in a state in which hydrogen exists in the oxidizing-agent electrode from shutdown of the system to the next starting time.    USE - Fuel cell system is used for electric-power generation in vehicles, and auxiliary machines used for electric-power generation.    ADVANTAGE - The fuel cell system prevents feeding of high-concentration hydrogen into the system downstream during starting. Performance degradation of fuel cell is prevented.    DETAILED DESCRIPTION - A fuel cell system consists of a fuel cell, a system stop controller (30), and a hydrogen-concentration reduction unit. The fuel cell consists of a fuel electrode and an oxidizing-agent electrode arranged opposingly on both sides of an electrolytic membrane. A hydrogen-containing fuel gas is supplied to the fuel electrode, and an oxygen-containing oxidizing-agent gas is supplied to the oxidizing-agent electrode. The system stop controller stops driving of the system in a state in which hydrogen exists in the oxidizing-agent electrode. The reducing unit reduces the hydrogen concentration in a downstream from the oxidizing-agent electrode in a state in which hydrogen exists in the oxidizing-agent electrode from shutdown of the system to the next starting time. An INDEPENDENT CLAIM is included for control method of fuel cell system.    DESCRIPTION OF DRAWING(S) - The drawing shows a block diagram of the fuel cell system. (Drawing includes non-English language text)    Fuel cell stack (1)    Load apparatus (2)    Air compressor (21)    Oxidizing-agent gas supply flow path (22)    System stop controller (30)