A grid spring is provided with first raised pieces that generate an elastic force for pressing a separator toward a power generation cell and second raised pieces that generate an elastic force independently of the first raised pieces. The spring constant of the first raised pieces decreases as a result of heating of a grid spring. The grid spring functions as a high reaction force spring as a result of a larger spring constant of the first spring member relative to a spring constant of the second spring member before heating. After being heated, the grid spring functions as a low reaction force spring as a result of the smaller spring constant of the first spring member before being heated.