A device for decreasing hydrogen concentration of a fuel cell system is installed in an exhaust system of a fuel cell system so as to discharge exhaust gas including hydrogen and air from fuel cells to the atmosphere through an exhaust line. The device includes: a catalyst diluter that includes catalysts for diluting hydrogen in the exhaust gas by generating a catalytic reaction, the catalyst diluter being connected to the exhaust line; and an air diluter that is disposed outside the catalyst diluter and guides external air to a gas exit side of the catalyst diluter, in which the catalyst diluter may include a valve unit that opens and closes an external air channel of the air diluter in accordance with flow pressure of the exhaust gas.