NOVELTY - Chemical reactor includes a reformer (3) for reforming an organic raw material into reformed gas. An inlet flow channel is to which a vessel is removably attached to enable the vessel to be connected with the reformer. An inlet port side cut-off valve (4) is disposed in the inlet flow channel and brought into an open state as the vessel is attached, thus permitting the organic raw material to pass through the inlet flow channel, and into a closed state as the vessel is removed, thus cutting off the inlet flow channel.    USE - For use in fuel cell system, modified ultramicro-gas turbine system, analyzing system, or modified gas turbine system.    ADVANTAGE - The chemical reactor and the fuel cell system are capable of inhibiting the leakage of the reacted gas and the leakage of a gas containing a by-product.    DETAILED DESCRIPTION - Chemical reactor comprises a vessel for housing an organic raw material having a higher saturated vapor pressure than atmospheric pressure. A reformer reforms the organic raw material into a reformed gas. An inlet flow channel is provided to which the vessel is removably attached to enable the vessel to be connected with the reformer. An inlet port side cut-off valve is disposed in the inlet flow channel and to be brought into an open state as the vessel is attached, thus permitting the organic raw material to pass through the inlet flow channel, and brought into a closed state as the vessel is removed, thus cutting off the inlet flow channel. An INDEPENDENT CLAIM is included for a fuel cell system comprising the chemical reactor, a fuel cell unit (5) for generating electrical energy by making use of the hydrogen-containing gas having carbon monoxide removed from the gas and air including oxygen. A combustion device burns the exhaust gas discharged from the fuel cell unit.    DESCRIPTION OF DRAWING(S) - The figure is a block diagram illustrating the construction of the fuel cell system.    Reformer (3)    Inlet port side cut-off valve (4)    Fuel cell unit (5)    Insulating member (13)    System main body (100)