NOVELTY - A fuel cell (1) comprises airtight solid electrolyte body (3) which conducts oxygen ion, negative electrode (4) which is formed on 1 side of solid electrolyte body and oxidizes reducible gas to oxidant gas during discharge, positive electrode (2) which is formed on other side of solid electrolyte body and reduces oxygen to oxygen ion during discharge, negative electrode fuel substance body (5) which reacts with oxidant gas, produces reducible gas, and becomes an oxide itself, and heating part (7) for performing heating maintenance of solid electrolyte body.    USE - A fuel cell for power supply of moving bodies, such as object for installation or motor vehicle, and portable power supply.    ADVANTAGE - Fuel cell is compact, can heat efficiently, and can maintain sealing property of enclosed space of gas in high-temperature region due to its airtight part, pressure of enclosed space of gas during operation can be maintained at a predetermined range, and organic compound and water thrown into enclosed space from the outside can produce electric power as fuel.    DETAILED DESCRIPTION - A fuel cell (1) comprises airtight solid electrolyte body (3) which conducts oxygen ion, negative electrode (4) which is formed on 1 side of solid electrolyte body and oxidizes reducible gas to oxidant gas during discharge, positive electrode (2) which is formed on other side of solid electrolyte body and reduces oxygen to oxygen ion during discharge, negative electrode fuel substance body (5) which reacts with oxidant gas, produces reducible gas, and becomes an oxide itself, heating part (7) for performing heating maintenance of solid electrolyte body and negative electrode fuel substance body more than predetermined temperature, and airtight part which forms sealed space (13) which is installed in solid electrolyte body and seals negative electrode and negative electrode fuel substance body with solid electrolyte body and heating part and in which repetition opening and closing is possible. When sealing sealed space through heat-resistant rubber or organic substance gasket and working the fuel cell, the distance (L) between heat generating body (8) of heating part and airtight part is spaced apart so that helium leakage rate of sealed space when working fuel cell is maintained less than or equal to 1x 10-2 Pa.m3/s. An INDEPENDENT CLAIM is included for a fuel cell system.    DESCRIPTION OF DRAWING(S) - The drawing shows a cross-sectional diagram of a fuel cell.    Fuel cell (1)    Positive electrode (2)    Airtight solid electrolyte body (3)    Negative electrode (4)    Negative electrode fuel substance body (5)    Heating part (7)    Heat generating body (8)    Sealed space (13)