A power supply apparatus comprising a plurality of planar fuel cell assemblies is disclosed. Each planar fuel cell assembly comprises two fuel cell members, a channel-forming member interposed between the two fuel cell members and defining a first channel for flowing a fluid fuel along with the two fuel cell members, and a coupling member to be coupled with an adjacent planar fuel cell assembly to define a second channel for flowing an ambient air, wherein the coupling member has a plurality of openings for flowing the ambient air therethrough.