A fuel cell includes a diffusion layer formed in a sheet having a small thickness. The fuel cell is assembled with a tension imposed on the sheet in an in-plane direction of the sheet. The tension is imposed on the diffusion layer from a frame disposed outside the fuel cell. The tension is of such a magnitude as to prevent a portion of the sheet positioned corresponding to a gas passage of a separator from being deformed when the fuel cell is assembled and has a tightening force thereby applied thereto. The tension is adjustable according to a fuel cell operating condition. These structures can prevent a separator rib from pushing into a membrane, maintaining the gas passability of the diffusion layer well.