A method of measuring concentration of a fuel is provided. First, a fuel cell unit having at least an anode, a cathode, and a membrane electrode assembly (MEA) is provided. Next, a fuel is supplied to the anode, while a reactive gas is supplied to the cathode. Then, the amount of the reactive gas supplied to the cathode is adjusted and the concentration of the fuel is estimated in accordance with the consumption rate of the reactive gas in the fuel cell unit, wherein a method of estimating the concentration of the fuel in accordance with a consumption rate of the reactive gas in the cathode includes measuring a concentration of the reactive gas supplied to the cathode and estimating the concentration of the fuel in accordance with a relationship between the concentration of the reactive gas supplied to the cathode and time.