A system in which a fuel cell can be operated with a fuel always having an optimal concentration is provided, and a time period necessary to start the fuel cell is reduced. A low concentration fuel mainly performing power generating reaction and a high concentration fuel mainly performing a power generating reaction and a reaction of raising the temperature of a power generation cell are individually stored in separate storage containers. This enables the fuels to be supplied to a negative electrode can be instantaneously switched depending on the temperature of the power generation cell. As a result, a fuel cell can be operated with a fuel always having an optimal concentration. Further, since the temperature of the power generation cell can be raised in a short time, the capacity of an auxiliary battery used when the temperature of the power generation cell is low can be reduced, and this enables a fuel cell system to be reduced in size.