A computer system of this invention includes a power input terminal which is
provided
outside a personal computer, and an external fuel cell assembly connected to the
power input terminal. Hence, a computer system in which water produced from the
fuel cell assembly is prevented from entering the computer can be provided. A personal
computer for which an operation mode for deriving the personal computer by a fuel
cell assembly is prepared to prevent any trouble due to user's misunderstanding
can be provided.