A portable computer including, a CPU operable in a maximum performance mode or in a low power consuming mode according to circumstances, supplied power from a fuel cell, the portable computer further including: a primary fuel valve enabling the fuel cell to be supplied fuel from an external source; a signal sensor sensing an external fuel supplying signal from the primary fuel valve; and a controller controlling the CPU to be operated in the maximum performance mode based on the sensed external fuel supplying signal. Thus, the CPU and the peripheral device is operated in the maximum performance mode when the portable computer receives the power from the external fuel tank, and operated in the low power consuming mode when receiving the power from the fuel cartridge in the fuel cell, thereby enabling maximum use time of the fuel cell. Furthermore, the portable computer which can use both of the AC/DC power and the fuel cell as a power source outputs the external power input signal generated when it receives the power from the AC/DC adapter or the fuel from the fuel tank and is operated in the maximum performance mode.