Maximum allowable load values of a welded portion in respective fracture modes of a load fracture, a moment fracture, and a nugget interior fracture are found based on at least one of a sheet thickness t, a tensile strength TS, an elongation El, and a chemical composition of a nugget portion in each of spot-welded steel sheets, a nugget diameter d of a welded portion, an effective width B of the welded portion determined by a distance between adjacent welded portions, edges or ridge lines, and a sectional height H. Then, according to these fracture modes, an allowable load value at every moment after the maximum allowable load value of the welded portion is reached is found, and a displacement or a time at which the allowable load value becomes 0, that is, at which a complete fracture occurs is found, thereby finding a fracture limit.