A fuel cell system comprising: a fuel cell generating an electric power by a reformed gas and an oxidant gas; an oxygen treatment device (11a6) being supplied with a source material from a supply source (Gs) to remove oxygen contained in the source material by a catalyst for oxygen treatment; a desulfurization device (11a8) being supplied with the source material after the oxygen in the source material is removed by the oxygen treatment device (11a6) to remove a sulfur component contained in the source material by a desulfurization catalyst; a reforming portion generating the reformed gas by the source material which is supplied from the desulfurization device (11a8) and in which the sulfur component is removed and water vapor, the reforming portion supplying the reformed gas to the fuel cell; and a control unit at least controlling the fuel cell, characterized by an oxygen concentration detection device (11a7) detecting a concentration of the oxygen contained in the source material, said oxygen concentration detection device (11a7) being a temperature sensor arranged at the oxygen treatment device (11a6) for detecting a temperature (Th) within the oxygen treatment device (11a6); and an evaporating portion generating the water vapor from water for reforming, wherein the control unit includes: a storage portion storing a first map that indicates a first correlation between the concentration of the oxygen contained in the source material and a type of the source material; an estimation portion estimating the type of the source material based on the concentration of the oxygen detected by the oxygen concentration detection device (11a7) in accordance with the first map; and an operation condition calculation portion calculating an operation condition of the fuel cell system depending on the type of the source material estimated by the estimation portion. a fuel cell generating an electric power by a reformed gas and an oxidant gas; an oxygen treatment device (11a6) being supplied with a source material from a supply source (Gs) to remove oxygen contained in the source material by a catalyst for oxygen treatment; a desulfurization device (11a8) being supplied with the source material after the oxygen in the source material is removed by the oxygen treatment device (11a6) to remove a sulfur component contained in the source material by a desulfurization catalyst; a reforming portion generating the reformed gas by the source material which is supplied from the desulfurization device (11a8) and in which the sulfur component is removed and water vapor, the reforming portion supplying the reformed gas to the fuel cell; and a control unit at least controlling the fuel cell, a storage portion storing a first map that indicates a first correlation between the concentration of the oxygen contained in the source material and a type of the source material; an estimation portion estimating the type of the source material based on the concentration of the oxygen detected by the oxygen concentration detection device (11a7) in accordance with the first map; and an operation condition calculation portion calculating an operation condition of the fuel cell system depending on the type of the source material estimated by the estimation portion.