PURPOSE: To enable molding of a thermoforming cap superior in strength easily, by performing thermoforming of a primary molded article obtained by providing a large diameter collar plate unitarily on the lower end of a cylinder wall of a topped cylinder, the collar plate is cut off and rounding processing of a cutting edge is performed by heating the same.

CONSTITUTION: Thermoforming of a primary molded article 1 is performed under a state wherein a large diameter collar 5 is provided unitarily on the lower end of a cylinder wall 3 of a topped cylinder. Then the collar plate 5 is cut off under a state wherein an inner circumferential side part of the same is left behind as a residual collar piece 7 and rounding processing is applied to the residual collar piece 7 through heating further. As for a rounding processing device, it is performed through heating while the primary molded article 1 and a heating roller 13 are abutted against a cutting edge under a state wherein the primary molded article 1 and the heating roller 13 are rotated mutually by making use of the heating roller 13 on the circumferential surface of which a circumferential groove 14 has been engraved. As the fringe of the residual collar piece 7 of the lower end of the cylinder wall 3 is rounded easily and a thickness of the residual collar piece 7 becomes thick through rounding like this, strength of an opening part is secured.