A fuel cell system includes at least one electricity generating unit for generating electric energy through an electrochemical reaction between oxygen and hydrogen, a reformer for reforming fuel to generate hydrogen gas to be supplied to the electricity generating unit, a fuel supply apparatus for absorbing a liquid fuel stored in a fuel reservoir and supplying the fuel to the reformer and an oxygen supply source for supplying oxygen to the electricity generating unit. The fuel supply apparatus employs an absorber with capillary channels in which osmotic pressure is produced due to a concentration differential caused by thermal energy. The osmotic pressure allows the fuel to flow.