A fuel cell system having a fuel cell that generates power in response to an electrochemical reaction between hydrogen and oxygen contained in air, and a compressor that supplies air to the fuel cell, in which an idle stop is executed to stop power generation by the fuel cell when a required load falls to or below a predetermined value, and during the idle stop, air is supplied in accordance with a voltage condition between a cathode and a anode of the fuel cell, regardless of the required load, characterized in that the fuel cell system comprises: a voltage detection unit that is configured to detect a cell voltage or a cell group voltage; a cell voltage calculation unit that is configured to calculate the voltage condition on the basis of a detection result from the voltage detection unit; a calculation result determination unit that is configured to determine, on the basis of calculation result from the cell voltage calculation unit, whether a previous value of an air supply amount supplied intermittently to a cathode during the idle stop is excessive, insufficient, or appropriate; a supplied air amount determination unit that is configured to decide to reduce the air supply amount relative to a preset fixed value or a previous supply amount if a determination result from the calculation result determination unit is excessive, to increase the air supply amount if the determination result is insufficient, and to maintain the current condition if the determination result is appropriate; an integrated supplied air amount detecting means for detecting an integrated amount of the air supplied to the fuel cell; and an end determining means for determining an end timing of the air supply performed during the idle stop on the basis of a detection value from the integrated supplied air amount detecting means, wherein a controller is configured to start the air supply during the idle stop when the integrated air supply amount is lower than a predetermined value and to stop the air supply during the idle stop when the integrated air supply amount equals or exceeds the predetermined value, and wherein the predetermined value is a value that satisfies two conditions, a first condition being an amount of air required to restore the voltage of the fuel cell to a voltage at which the response delay when returning from an idle stop can be suppressed and a second condition being an amount of air at which variation among the single cells in a voltage decrease speed can be suppressed. a voltage detection unit that is configured to detect a cell voltage or a cell group voltage; a cell voltage calculation unit that is configured to calculate the voltage condition on the basis of a detection result from the voltage detection unit; a calculation result determination unit that is configured to determine, on the basis of calculation result from the cell voltage calculation unit, whether a previous value of an air supply amount supplied intermittently to a cathode during the idle stop is excessive, insufficient, or appropriate; a supplied air amount determination unit that is configured to decide to reduce the air supply amount relative to a preset fixed value or a previous supply amount if a determination result from the calculation result determination unit is excessive, to increase the air supply amount if the determination result is insufficient, and to maintain the current condition if the determination result is appropriate; an integrated supplied air amount detecting means for detecting an integrated amount of the air supplied to the fuel cell; and an end determining means for determining an end timing of the air supply performed during the idle stop on the basis of a detection value from the integrated supplied air amount detecting means, wherein a controller is configured to start the air supply during the idle stop when the integrated air supply amount is lower than a predetermined value and to stop the air supply during the idle stop when the integrated air supply amount equals or exceeds the predetermined value, and wherein the predetermined value is a value that satisfies two conditions, a first condition being an amount of air required to restore the voltage of the fuel cell to a voltage at which the response delay when returning from an idle stop can be suppressed and a second condition being an amount of air at which variation among the single cells in a voltage decrease speed can be suppressed.