NOVELTY - A fuel cell system comprises a fuel cell for generating power by electrochemical reaction between a fuel gas supplied to a fuel electrode and an oxidizer gas; a fuel gas supply channel (L1); a circulation channel (L2) through which a gas discharged from fuel electrode is fed to fuel gas supply channel; a circulator unit for circulating the fuel gas; a discharge channel (L3) through which a gas flowing in the circulation channel is discharged; a discharge mount-adjusting unit for adjusting an amount of an impurity to be discharged through the discharge channel; and a mode switching unit.    USE - A fuel cell system for providing electrical power to vehicle.    ADVANTAGE - The impurity content in the circulation channel and the fuel electrode can be controlled by taking into account the driving mode. As a result, compared to when the impurity content is controlled without considering the driving mode, the impurity content can be more effectively controlled in all driving modes. Thus, degradation in fuel efficiency under the normal power generation mode can be suppressed while improving the fuel efficiency under the idle mode.    DETAILED DESCRIPTION - A fuel cell system comprises:    (1) a fuel cell for generating power by electrochemical reaction between a fuel gas supplied to a fuel electrode and an oxidizer gas supplied to an oxidizer electrode;    (2) a fuel gas supply channel (L1) through which the fuel gas is supplied to the fuel electrode;    (3) a circulation channel (L2) through which a gas discharged from the fuel electrode of the fuel cell is fed to the fuel gas supply channel;    (4) a circulator unit provided in the circulation channel to circulate the fuel gas;    (5) a discharge channel (L3) through which a gas flowing in the circulation channel is discharged;    (6) a discharge mount-adjusting unit for adjusting an amount of an impurity to be discharged through the discharge channel;    (7) a mode switching unit for switching a driving mode between a normal power generation mode in which power required by the system is supplied through power generation in the fuel cell and an idle mode in which power generation in the fuel cell is stopped and power required by the system is supplied from a power storing unit;    (8) a setting unit for variably setting a target control value of an impurity content in circulation channel and/or fuel electrode based on the driving mode; and    (9) a controller for controlling the discharge mount-adjusting unit based on the target control value.    An INDEPENDENT CLAIM is included for a method for controlling a fuel cell system.    DESCRIPTION OF DRAWING(S) - The drawing is a block diagram showing an overall structure of a fuel cell system.    Fuel cell stack (1)    Voltage sensor (8)    Fuel gas supply channel (L1)    Circulation channel (L2)    Discharge channel (L3)    Closed-loop cooling channel (L6)