NOVELTY - A sensor (668) detects the alcohol concentration of a fuel such as an alcohol gas (124b) stored in a container (662). A fuel cell (100) has a solid-polymer electrolyte film (114) arranged between a fuel electrode (102) and an oxidizing agent electrode (108). The sensor has a polymer film (665) provided in the container. The concentration is detected based on change in proton conductivity of the polymer film.    USE - Fuel cell system with alcohol concentration measuring device (claimed), used as power supply unit of small portable equipment such as notebook computer, personal digital assistant, mobile telephone, various cameras, navigation system, and portable music player.    ADVANTAGE - Enables easily measuring the alcohol concentration of the alcohol gas, using the sensor having a simple structure.    DETAILED DESCRIPTION - The sensor has the electrode terminals (666,667) arranged at the polymer film. The concentration is calculated based on the measured resistance value between the terminals. A hydrophobic film covers the terminals. A portion of the solid-polymer electrolyte film is used as the polymer film. A gasification unit gasifies the liquid fuel (124a) comprising alcohol, so that gasified fuel is supplied to the container. The operation of the gasification unit is controlled based on the detected concentration. A liquid-fuel storage cartridge is removably fixed to the fuel cell. The alcohol concentration of the alcohol gas is corrected based on the measured temperature and the measured pH in the container. A warning is provided when the detected concentration deviates from a preset range.    INDEPENDENT CLAIMS are also included for the following:    (1) alcohol concentration measuring device with the sensor; and    (2) alcohol concentration measuring method which involves measuring the change in the proton conductivity of the polymer film arranged in the alcohol-gas container.    DESCRIPTION OF DRAWING(S) - The figure shows a block diagram of the fuel cell system. (Drawing includes non-English language text).    Liquid fuel (12a)    Alcohol gas (124b)    Fuel cell (100)    Fuel electrode (102)    Electrode (108)    Solid-polymer electrolyte film (114)    Container (662)    Polymer film (665)    Electrode terminals (666,667)    Sensor (668)