PURPOSE: To obtain the titled crown cap foamed uniformly and having uniform and excellent sealing property by foaming the crown cap with unfoamed liner in a heating oven and a high frequency heating device.

CONSTITUTION: The crown cap A with unfoamed liner is preheated by passing through the high frequency heating device 2, and the decomposition of the foaming agent in the inside 5 and the bottom 6 of the liner B is promoted. Then, the decomposition of the foaming agent in the surface 4 of the liner is promoted by passing through the heating oven 1. Fluctuation of the decomposition temperature among the surface 4, inside 5, bottom 6, i.e. in the liner as a whole becomes extremely small by the temperature control due to the matching of the high frequency heating device 2 and the heating oven 1.