A fuel cell system, comprising: a fuel cell; a voltage measuring portion that measures a voltage of the fuel cell; an electric current adjusting portion that adjusts an electric current flowing in the fuel cell; an electric current-voltage characteristic information obtaining portion (91 b) that controls the electric current adjusting portion (91 a) to change the electric current, and obtains electric current-voltage characteristic information that is information indicating a correspondence relation between an electric current value and a voltage value measured by the voltage measuring portion; and a negative voltage cause determining portion that determines, if the voltage of the fuel cell is a negative voltage, a cause of the negative voltage of the fuel cell, based on the obtained electric current-voltage characteristic information, wherein, in the fuel cell, a fuel gas containing hydrogen and an oxidant gas containing oxygen are used as reaction gases; characterized in that, when the electric current-voltage characteristic information is obtained, the electric current-voltage characteristic information obtaining portion obtains the voltage value while changing the electric current in a range in which the electric current is higher than 0; the negative voltage cause determining portion determines a zero current-time voltage value that is the voltage value at a time when the electric current is 0, based on the obtained electric current-voltage characteristic information, using extrapolation; if the zero current-time voltage value is lower than 0 volt, the negative voltage cause determining portion determines that the cause is insufficiency of an amount of hydrogen; if the zero current-time voltage value is equal to or higher than 0 volt and lower than an open circuit voltage of the fuel cell at a normal time, the negative voltage cause determining portion determines that the cause is insufficiency of an amount of oxygen; and if the zero current-time voltage value is equal to or higher than the open circuit voltage, the negative voltage cause determining portion determines that the cause is drying-up. a fuel cell; a voltage measuring portion that measures a voltage of the fuel cell; an electric current adjusting portion that adjusts an electric current flowing in the fuel cell; an electric current-voltage characteristic information obtaining portion (91 b) that controls the electric current adjusting portion (91 a) to change the electric current, and obtains electric current-voltage characteristic information that is information indicating a correspondence relation between an electric current value and a voltage value measured by the voltage measuring portion; and a negative voltage cause determining portion that determines, if the voltage of the fuel cell is a negative voltage, a cause of the negative voltage of the fuel cell, based on the obtained electric current-voltage characteristic information, in the fuel cell, a fuel gas containing hydrogen and an oxidant gas containing oxygen are used as reaction gases; when the electric current-voltage characteristic information is obtained, the electric current-voltage characteristic information obtaining portion obtains the voltage value while changing the electric current in a range in which the electric current is higher than 0; the negative voltage cause determining portion determines a zero current-time voltage value that is the voltage value at a time when the electric current is 0, based on the obtained electric current-voltage characteristic information, using extrapolation; if the zero current-time voltage value is lower than 0 volt, the negative voltage cause determining portion determines that the cause is insufficiency of an amount of hydrogen; if the zero current-time voltage value is equal to or higher than 0 volt and lower than an open circuit voltage of the fuel cell at a normal time, the negative voltage cause determining portion determines that the cause is insufficiency of an amount of oxygen; and if the zero current-time voltage value is equal to or higher than the open circuit voltage, the negative voltage cause determining portion determines that the cause is drying-up.