A vessel cap is comprised of a body mounted at a vessel inlet through which a liquid material stored in a vessel is discharged outwardly and having a certain space therein. A pressing plate is disposed in the body to be movable in upper and lower directions. A cutter is formed at an edge of a lower surface of the pressing plate in a circumferential direction for penetrating a sealing member when the pressing plate is pressed and which cuts the sealing member when the body is rotated. A hooking member downwardly protrudes at the lower surface of the pressing plate for storing the sealing member cut by the cutter in the body. A connection portion is formed between an outer circumferential surface of the pressing plate and an inner circumferential surface of the body for guiding the pressing plate to be moved in upper and lower directions.