NOVELTY - The system has a measurement flow path i.e. measurement piping (34), horizontally positioned with respect to ground. A flow volume detection unit detects a flow volume of hydrogen circulated via a hydrogen flow conduction path i.e. anode off-gas flow path. Opposing surfaces (41A, 42A) of an ultrasonic transmitter and an ultrasonic receiver have an area smaller than flow-path cross-sectional area of the measurement flow path. The transmitter and the receiver make a shaft center part of the measurement flow path to transmit an ultrasonic wave between the transmitter and the receiver.    USE - Fuel cell system for use in a vehicle (claimed) i.e. running drive motor vehicle.    ADVANTAGE - The system allows a moisture content to be circulated inside the hydrogen-flow conduction path in which hydrogen circulates so as to avoid the moisture content from adhering to the ultrasonic transmitter and the ultrasonic receiver, thus making the ultrasonic wave to be transmitted/received appropriately and preventing fall of detection accuracy of the flow volume of hydrogen, and hence improving reliability of a detection result. The flow volume detection unit detects the flow volume of hydrogen circulated via a hydrogen flow conduction path, so that presence or absence of abnormality such as open failure, of an opening-and-closing unit can be precisely detected, thus performing control of the vehicle based on the detection result with high reliability, and hence performing leak detection at quicker responsiveness with high precision and suppressing deterioration of the system.    DETAILED DESCRIPTION - The flow volume detection unit is an ultrasonic flow meter (24). The anode off-gas flow path is a hydrogen-flow delivery pipe (31). The ultrasonic transmitter and an ultrasonic receiver are combined to form ultrasonic transceivers (41, 42).    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of a hydrogen flow delivery pipe with which an ultrasonic flow meter of a fuel cell system is positioned.    Ultrasonic flow meter (24)    Hydrogen-flow delivery pipe (31)    Measurement piping (34)    Ultrasonic transceivers (41, 42)    Opposing surfaces (41A, 42A)