Disclosed is a closure device for a container, which seals an opening (6) that opens at the top of a container (2) and has a rim (7) on its periphery. The closure device comprises a seal member (8) that is to cover the opening (6) on the top of the container (2), and a synthetic resin cap (9) that is to press the seal member (8) against the upper surface of the rim (7). The seal member (8) comprises a flange (10) to be mounted on the upper surface of the rim (7). The cap (9) comprises a top wall (15), a cylinder part (16), and a locking part (18). The top wall (15) is to be disposed above the flange (10). The cylinder part (16) hangs down from the outer circumference of the top wall (15). The locking part (18) is disposed on the inner surface of the cylinder part (16). After the cap (9) is pushed down, the locking part (18) locks at the under surface of the rim (7). The upper surface of the flange (10) has a compressible projection (29). The compressible projection (29) is annularly arranged in contact with and along the outer circumference of the flange (10) in a planar view.