NOVELTY - A fuel cell system has an impurity concentration detecting device (33) for estimating a value related to an impurity concentration (Cn) in a fuel electrode. An idling-stop controller controls idling-stop of a fuel cell based on power requirement to the fuel cell. The idling-stop controller prohibits idling-stop based on the value when the impurity concentration exceeds threshold. The fuel cell generates power by reaction of a fuel gas, and has an inlet through which the fuel gas is supplied to the fuel electrode.    USE - Used as an electrochemical device for converting chemical energy of fuel gas, e.g. hydrogen gas, and oxidizing gas containing oxygen electrochemically into electrical energy.    ADVANTAGE - The idling-stop controller prohibits idling-stop based on the value that relates to the impurity concentration, thus preventing deterioration of a catalyst in the fuel electrode. The arrangement of the system effectively increases excess supply ratio of hydrogen gas to the fuel cell.    DETAILED DESCRIPTION - A fuel cell system comprises:    (i) a fuel cell which generates power by a reaction of a fuel gas, the fuel cell having a fuel electrode to which the fuel gas is supplied for the reaction, an inlet through which the fuel gas is supplied to the fuel electrode, and an outlet through which the fuel gas unused in the reaction is discharged from the fuel electrode;    (ii) a fuel gas circulation system for re-circulating the unused fuel gas from the outlet to the inlet;    (iii) an impurity concentration detecting device which detects or estimates a value related to an impurity concentration in the fuel electrode or the fuel gas circulation system; and    (iv) an idling-stop controller which controls an idling-stop of the fuel cell, in which power generation of the fuel cell is temporarily suspended, based on a power requirement to the fuel cell, wherein the idling-stop controller permits or prohibits the idling-stop based on the value detected or estimated by the impurity concentration detecting device.    An INDEPENDENT CLAIM is also included for controlling idling-stop of a fuel cell system.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic representation of a fuel cell system.    Impurity concentration (Cn)    Fuel cell stack (2)    Circulation pump (11)    Purge valve (12)    Impurity concentration detecting device (33)