To provide a heat sealed part free of damage to sealed-part surfaces due to heat and provide a highly-speedy, steady, consecutive heat seal method which prevents resin from sticking to a heat plate, in the heat sealing of polyvinyl alcohol films.

In the part where polyvinyl alcohol films are sealed together, the sealed part is free of thermal history on the outsides of the films which is caused by fusion hardening, and the joined faces of the films are fused together. The sealed part is obtained by using the polyvinyl alcohol films, by applying polyvinyl alcohol plasticizer to the joined insides of the part to be sealed, by sealing the part under heat while preventing the presence of the plasticizer on the outsides of the films, thereby fusing the joined insides without imparting thermal history due to the fusion hardening to the outsides of the heat-sealed part.