A lid (11) is thermally fused to a flange (10b) of a cup container (1) and is provided with: an outer layer film (21) that is molecularly aligned by biaxial stretching; an inner layer film (23) that is laminated on the outer layer film (21) and that is thermally fusible to the flange (10b); one or more alignment relaxation parts (21a) that are formed in the outer layer film (21); and a breaking part (13) that is formed at a portion where the one or more alignment relaxation parts (21a) face each other with a predetermined interval therebetween and that is broken by an increase in the packaging inner pressure caused by an microwave oven. The opening strength of the breaking part (13) is smaller than the sealing strength at which the flange (10b) is thermally fused.

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