A structure for mounting a fuel cell stack in an enclosure or a frame includes a first mounting mechanism for fastening and mounting a first mounting part located at a first side of the fuel cell stack in a longitudinal direction of the stack, which is a cell stacking direction, to the enclosure or the frame in a completely fixing fashion, and a second mounting mechanism for mounting a second mounting part located at a second side of the fuel cell stack in the longitudinal direction of the stack to the enclosure or the frame in a state of being movable in the longitudinal direction of the stack.