NOVELTY - Operation method of a fuel cell system, comprises: providing a stopping state of the cell system with first stage stopping state for stopping a power generation of the system while remaining hydrogen in the hydrogen line and a second stage stopping state for substituting the hydrogen in the hydrogen line with air; and transferring second stage stopping state by way of first stage stopping state, while the cell voltage of fuel cell in the first stage stopping state, in a case where the voltage of cell increases to greater than or equal to 0.2 V per one cell, transfers the system from first stage to second stage.    USE - The method is useful for the operation of a fuel cell system, preferably frequently conducting starting and stopping, since the number of cycles of purging hydrogen in the hydrogen line and releasing the hydrogen to the outside is increased.    ADVANTAGE - The power generation efficiency can be improved, due to using hydrogen as the fuel gas with no or small loss for the power generation. The method has improved safety, and shortened starting and stopping times.    DETAILED DESCRIPTION - Operation method of a fuel cell system having a fuel cell (1), a hydrogen line for supplying and discharging hydrogen to and from the fuel cell, a hydrogen inlet valve (2) disposed to the inlet portion of the fuel cell in the hydrogen line, a hydrogen exit valve (3) disposed to the exit portion of the fuel cell in the hydrogen line, an air introduction valve for supplying air to the hydrogen line, a hydrogen pressure sensor (4) disposed to the inlet portion of the fuel cell in the hydrogen line, an air line for supplying and discharging air to and from the fuel cell, an air inlet valve (5) disposed to the inlet portion of the fuel cell in the air line, an air pressure sensor disposed to the inlet portion of the fuel cell in the air line, a temperature sensor for measuring the temperature of the fuel cell, a power line for outputting the electric power generated by the fuel cell, a load and a small adjusting load connected to the output line of the fuel cell, and a control apparatus for controlling the operation of auxiliary equipment e.g. sensors and the valves and an amount of current flowing to the loads, comprises: providing a stopping state of the fuel cell system with a first stage stopping state for stopping a power generation of the fuel cell system while remaining hydrogen in the hydrogen line and a second stage stopping state for substituting the hydrogen in the hydrogen line with air; and transferring second stage stopping state by way of the first stage stopping state, while the cell voltage of the fuel cell in the first stage stopping state, in a case where the voltage of the cell increases to greater than or equal to 0.2 V per one cell, transfers the fuel cell system from the first stage stopping state to the second stage stopping state.    DESCRIPTION OF DRAWING(S) - The figure shows the fuel cell system.    Fuel cell stack (1)    Hydrogen inlet valve (2)    Hydrogen exit valve (3)    Hydrogen pressure sensor (4)    Air inlet valve (5)