A fuel cell stack, comprising: a plurality of cell modules each comprising an integrally stacked plurality of single cells; a sealing plate intervened between cell modules (M, M) of the plurality of cell modules; a manifold that penetrates the plurality of cell modules and the sealing plate(s) in a stacking direction to distribute reaction gas, wherein each single cell of the plurality of single cells comprises a membrane electrode assembly with a peripheral frame and a pair of separators (2A, 2B) holding the peripheral frame and the membrane electrode assembly between them, the peripheral frame and the separators (2A, 2B) of each single cell and the sealing plate have a respective distribution hole that continues to each other in a stacked state to form the manifold, the sealing plate comprises a sealing member that is disposed around the manifold between the sealing plate and the cell module to seal the manifold, the sealing member comprises an extended portion that extends toward the manifold so that an end face of the extended portion is flush with an inner wall of the manifold, and a flattening face of the peripheral frame, a flattening face of the extended portion, and flattening faces (F2, F3) of the separators (2A, 2B) continue to be flush with each other in the inner walls of the respective distribution hole and form the inner wall of the manifold so that the inner wall of the manifold is formed in a continuous flat shape that extends in the stacking direction of the single cells. a plurality of cell modules each comprising an integrally stacked plurality of single cells; a sealing plate intervened between cell modules (M, M) of the plurality of cell modules; a manifold that penetrates the plurality of cell modules and the sealing plate(s) in a stacking direction to distribute reaction gas, wherein each single cell of the plurality of single cells comprises a membrane electrode assembly with a peripheral frame and a pair of separators (2A, 2B) holding the peripheral frame and the membrane electrode assembly between them, the peripheral frame and the separators (2A, 2B) of each single cell and the sealing plate have a respective distribution hole that continues to each other in a stacked state to form the manifold, the sealing plate comprises a sealing member that is disposed around the manifold between the sealing plate and the cell module to seal the manifold, the sealing member comprises an extended portion that extends toward the manifold so that an end face of the extended portion is flush with an inner wall of the manifold, and a flattening face of the peripheral frame, a flattening face of the extended portion, and flattening faces (F2, F3) of the separators (2A, 2B) continue to be flush with each other in the inner walls of the respective distribution hole and form the inner wall of the manifold so that the inner wall of the manifold is formed in a continuous flat shape that extends in the stacking direction of the single cells. wherein each single cell of the plurality of single cells comprises a membrane electrode assembly with a peripheral frame and a pair of separators (2A, 2B) holding the peripheral frame and the membrane electrode assembly between them, the peripheral frame and the separators (2A, 2B) of each single cell and the sealing plate have a respective distribution hole that continues to each other in a stacked state to form the manifold, the sealing plate comprises a sealing member that is disposed around the manifold between the sealing plate and the cell module to seal the manifold, the sealing member comprises an extended portion that extends toward the manifold so that an end face of the extended portion is flush with an inner wall of the manifold, and a flattening face of the peripheral frame, a flattening face of the extended portion, and flattening faces (F2, F3) of the separators (2A, 2B) continue to be flush with each other in the inner walls of the respective distribution hole and form the inner wall of the manifold so that the inner wall of the manifold is formed in a continuous flat shape that extends in the stacking direction of the single cells.