PURPOSE: To effect heat seal in such a manner as to meet the container requirements such as an excellent sealing, an easy cover opening and prevention of cover gathering by forming doubly annular downward projections, inner and outer, corresponding to the flange of a container and making the lower end faces of both projections flat for use as pressure applying face and the end face of the inner projection lower than that of the outer projection.

CONSTITUTION: A container 1 is placed in a cavity 21 of a lower jig 20, a flange 2 thereof is placed on a pressure receiving face 22, and a cover 3 is placed on a container 1. The flange 2 and the cover 3 are compressed by the pressure receiving face 22 of the lower jig 20 and by the pressure applying faces of both projections 12 and 13 of an upper jig 10. The upper jig 10 is heated by heaters 14 and 23 to heat seal the flange 2 and the cover 3 together. At a portion where the inner projection 12 exists, a firm heat seal is effected to satisfy the requirement of an excellent sealing and at a portion where the outer portion 13 exists, a light heat seal is effected to such an extent as to be able to prevent the gathering of the cover 3.