PROBLEM TO BE SOLVED: To provide a simple closure device for a vial, which ensures sealing of the opening of the vial with a seal member despite size variations in the overall height of the vial, the thickness of a flange of the seal member, or the like.SOLUTION: The closure device comprises the seal member (8) that is to cover the opening (6) on the top of the vial (2), and a synthetic resin cap (9) that is to press the seal member (8) against the upper surface of the rim (7). The cap (9) comprises a top wall (15) disposed above the flange (10), a cylinder part (16) hanging down from the outer circumference of the top wall (15), and a locking part (18) disposed on the inner surface of the cylinder part (16). The seal member (8) comprises a flange (10) to be mounted on the upper surface of the rim (7). The upper surface of the flange (10) has a compressible projection (29) annularly arranged in contact with and along the outer circumference of the flange (10) in a planar view.
