A method for manufacturing a multiple-wall synthetic-resin case having vertical multiple walls. The case is manufactured by integrating both upper and lower halves into one piece by jointing together; more specifically, each of the vertical walls is manufactured by jointing the upper wall formed in the upper half to the corresponding lower wall formed in the lower half. The jointing portions of the respective pairs of mating vertical walls are provided with different elevations so that they form their individual closed loops on the respective planes of elevations different from each other, conductive wire is inserted between the jointing surfaces of the upper and lower wall halves at each mating portion and apply high-frequency-heating to the conductive wire for welding the jointing surfaces.