A fuel cell system includes cells (40) in form of rectangular plates stacked in a spaced relationship to form a cell assembly (41), with intercellular passages (y) formed between the cells. Each cell is held by a pair of cell holders (5) and includes an electrolyte layer (1) having an oxygen electrode (2) applied to one surface thereof and a fuel electrode (3) applied to the other surface, and a passage-defining member (4) for defining intracellular passages (x) opposed to the oxygen electrode (2) or the fuel electrode (3). The intracellular passages (x) are open at one pair of opposite sides and closed at the other pair of opposite sides of each cell. The intercellular passages (y) are closed at open sides (40b) where the intracellular passages (x) are open, and open at closed sides (40c) where the intracellular passages (x) are closed. The cell holders (5) define notches (51) for receiving the opposite sides (40a) where the intracellular passages (x) are open. The notches (51) each include contact surfaces (53) for tight contact with the closed sides (40c) adjacent opposite ends of each of the open sides (40a), each of the notches having substantially the same depth as a thickness of one of the cells. The cell assembly (41) is formed by the cells (40) stacked with the open sides (40a) fitted into the notches (51) of opposed pairs of the cell holders (5). <MATH>
