NOVELTY - A fuel cell system is controlled by keeping a gas mixture at a higher pressure than a separated reformer gas; supplying the reformer gas to an anode side, and an oxidation agent to a cathode side of the module, such that fluids on the anode and cathode sides are separated by a separation diaphragm unit; and holding a pressure differential between a side of the reformer unit facing the anode and cathode sides below a predefined value.    USE - For controlling a fuel cell system.    ADVANTAGE - The method avoids mechanical damage to the fuel cell, and prevents diaphragm electrode unit from bursting or being contaminated.    DETAILED DESCRIPTION - Control of a fuel cell system (2), in which hydrogen-containing reformer gas is produced in a reformer unit    (1) by selectively separating the reformer gas from a gas mixture using a diaphragm module (4) having a diaphragm, involves keeping the gas mixture at a higher pressure than the separated reformer gas during normal operation of the fuel cell system; supplying the reformer gas to an anode side (18) of a fuel cell module and oxidation agent to a cathode side (19) of the fuel cell module, such that the fluids on the anode side and the cathode side of the fuel cell module are separated by a separation diaphragm unit; and holding a pressure differential between a side of the reformer unit facing the anode side and the cathode side of the fuel cell module below a predefined value during abnormal operation including bursting of the diaphragm. An INDEPENDENT CLAIM is also included for a fuel cell system comprising a reformer unit for producing a hydrogen-containing reformer gas using a diaphragm module having a diaphragm; and a fuel cell module having fuel cell(s), each having an anode side and a cathode side separated from each other by a separation diaphragm unit.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic view of the fuel cell.    Reformer unit (1)    Fuel cell system (2)    Diaphragm module (4)    Low-pressure area (16)    Anode side (18)    Cathode side (19)    Pressure relief valve (29)