A reduction process is performed to each fuel electrode layer by supplying a reduction gas into each fuel channel 22 in the state in which a perimetric portion of a sheet body 11 is held to be sealed by perimetric portions of an upper support member 122 and a lower support member 121. In the case of a small-sized fuel cell in which the thickness of the sheet body 11 is 20˜500 μm, the fuel electrode layer is greater in thickness than the solid electrolyte layer and the air electrode layer, and the area of the orthogonal projection of the plane portion 12a of each support member 12 is 1˜100 cm2, a ratio of a warpage of not more than 0.05 cm−1 on the sheet body with respect to the area of the orthogonal projection can be achieved at room temperature after the reduction process.