A method for assembling fuel cell stack comprises the steps of alternately stacking the prescribed number of fuel cells and bipolar plates to form a plurality of substacks, arranging the laid substacks on a supporting base so that the central axes of the substacks parallel to the stacking direction of the fuel cell stack are on a horizontal straight line to form a complete fuel cell stack, tying up the fuel cell stack to apply a predetermined compressive load to the fuel cell components, making the fuel cell stack vertical, and then removing said supporting base from the fuel cell stack. The supporting base on which a plurality of fuel cell substacks is provided at its both end with a pair of side plates, and a pressure plate is slidably arranged on the base and connected to driving means.