NOVELTY - Gas leak detection device comprises a supply-amount detector; consumption-amount calculator that calculates an amount of fuel gas consumed by the fuel cell; pressure detector (310, 340); change-amount calculator that obtains a change in the pressure of the fuel gas; difference-amount calculator; and fuel-gas leak detector that determines that a fuel gas leak is present when the difference is equal to or greater than a first predetermined value.    USE - For detecting fuel gas leak in a fuel cell system (claimed).    ADVANTAGE - The device accurately and easily detects a fuel gas leak. It prevents a reduction of the gas leak detection accuracy due to an increase in the output of the fuel cell, which avoids an error in detecting a fuel gas leak.    DETAILED DESCRIPTION - Gas leak detection device that detects a gas leak in a fuel cell system comprises:    (a) a supply-amount detector provided in the supply passage, where the supply passage includes a first passage positioned downstream of the supply-amount detector, the fuel cell includes a second passage through which the fuel gas flows, and the discharge passage includes a third passage positioned upstream of the purge valve;    (b) consumption-amount calculator that calculates an amount of fuel gas consumed by the fuel cell;    (c) pressure detector that detects a pressure of the fuel gas in a gas leak detection passage including the first passage, the second passage, and the third passage;    (d) change-amount calculator that obtains a change in the pressure of the fuel gas using the detected pressure, and calculates a change in an amount of fuel gas in the gas leak detection passage due to the change in the pressure of the fuel gas;    (e) difference-amount calculator that calculates a difference between the detected amount of supplied fuel gas and a sum of the amount of consumed fuel gas and the change in the amount of fuel gas in the gas leak detection passage by subtracting the amount of consumed fuel gas and the change in the amount of fuel gas in the gas leak detection passage from the amount of supplied fuel gas; and    (f) fuel-gas leak detector that determines that a fuel gas leak is present in the gas leak detection passage when the difference is equal to or greater than a first predetermined value.    INDEPENDENT CLAIMS are also included for:    (i) fuel cell system (100) comprising a fuel cell; fuel gas supply source; supply passage through which fuel gas is supplied from the fuel gas supply source to the fuel cell; discharge passage through which fuel gas is discharged from the fuel cell; purge valve provided in the discharge passage, which adjusts an amount of fuel gas discharged from the fuel cell; supply-amount detector provided in the supply passage, which detects a fuel gas supply amount that is an amount of supplied fuel gas; pressure detector that detects a pressure of the fuel gas in a gas leak detection passage; consumption-amount calculator; change-amount calculator; difference-amount calculator; and a fuel- gas leak detector; and    (ii) detecting a gas leak in a fuel cell system comprising detecting, at a predetermined first position within the supply passage, an amount of supplied fuel gas; calculating an amount of fuel gas consumed by the fuel cell; detecting a pressure of the fuel gas in a gas leak detection passage; calculating a change in an amount of fuel gas in the gas leak detection passage due to the change in the pressure of the fuel gas; calculating a difference between the detected amount of supplied fuel gas and a sum of the amount of consumed fuel gas and the change in the amount of fuel gas in the gas leak detection passage by subtracting the amount of consumed fuel gas and the change in the amount of fuel gas in the gas leak detection passage from the detected amount of supplied fuel gas; and determining that a fuel gas leak is present in the gas leak detection passage when the difference is equal to or greater than a predetermined value.    DESCRIPTION OF DRAWING(S) - The figure shows a block diagram showing the configuration of a fuel cell system.    Fuel cell system (100)    Circulation pump (250)    Hydrogen flow meter (300)    Pressure detectors (310,340)