When employing an intermittent supplying method, the supplying rate of thermoplastic sheets is increased while a welding force is reduced, in comparison with a case in which an anvil and a horn come close to and separate from each other at a predetermined cycle. In a state in which an anvil (16) rotates about a rotating shaft (J1), a plurality of partly-finished products (1Z) having side panel pieces (3 and 4) is intermittently supplied to the places between the welding surfaces of the anvil (16) and horns (25). Thus, in each of the partly-finished products (1Z), the respective side panel pieces (3) and (4) are continuously welded to each other with a predetermined pattern. The welding surfaces of the anvil (16) are provided over a whole circumference about the rotating shaft (J1) such that the interval between the welding surfaces of the anvil (16) and the horns (25) is maintained at a interval at which the respective side panel pieces (3 and 4) are continuously weldable to each other with the predetermined pattern regardless of a rotating position of the anvil (16).
