A fuel cell system includes first and second fuel cell stacks which are juxtaposed to each other. An assembly manifold is attached to the first and second fuel cell stacks. A connection block is provided at a central position of the assembly manifold. A fuel gas supply port and a fuel gas discharge port are provided on a front surface of the connection block, and an oxygen-containing gas supply port and an oxygen-containing gas discharge port are provided on a back surface of the connection block. A fuel gas and an oxygen-containing gas are equally supplied to each of the first and second fuel cell stacks.