A pressure vessel comprising: a boss portion comprising a boss extension portion having a cylindrical shape in which a through hole is formed at a central part to pass therethrough and a boss flange portion formed below the boss extension portion and integrally expanding outward in a radial direction along a circumferential direction in which the through hole extends in a vertical direction, wherein a sealing step protruding outward in a radial direction along a circumferential direction is formed on the boss flange portion; a liner portion having a container shape in which an accommodation space configured to communicate with the through hole is formed to accommodate a fluid therein while a top is insertion-injection molded and seal-coupled along a bottom surface of the boss flange portion, wherein a sealing groove facing a radially outer end of the sealing step is formed to be recessed upward below an inner end of the liner portion extending along the bottom surface of the boss flange portion; a sealing portion seal-coupled to a boundary area between a bottom of the boss flange portion and said top of the liner portion to block leakage of fluid accommodated in the accommodation space, wherein the sealing portion comprises a sealing plate and a sealing shape-matching portion integrally extending upward from a top of the sealing plate, said sealing plate surrounding a radial outside of the through hole along a circumferential direction and having an inner top surface arranged radially inward in radial direction and in surface contact with the bottom surface of the sealing step, and an outer top surface arranged radially outward in radial direction and in surface contact with a bottom surface of a liner extension portion formed above the liner portion, and said sealing shape-matching portion having a radially inner surface in surface contact with the outer end of the sealing step and a top surface pressed against and shape-matched with the sealing groove; characterized in that a first sealing protrusion and a third sealing protrusion extend upward from said top surfaces (31a, 31b) of the sealing plate, and a second sealing protrusion is formed on a top surface of the sealing shape-matching portion and integrally extends and protrudes toward the sealing groove along a circumferential direction so that a top end is elastically pressurized toward the sealing groove. a boss portion comprising a boss extension portion having a cylindrical shape in which a through hole is formed at a central part to pass therethrough and a boss flange portion formed below the boss extension portion and integrally expanding outward in a radial direction along a circumferential direction in which the through hole extends in a vertical direction, wherein a sealing step protruding outward in a radial direction along a circumferential direction is formed on the boss flange portion; a liner portion having a container shape in which an accommodation space configured to communicate with the through hole is formed to accommodate a fluid therein while a top is insertion-injection molded and seal-coupled along a bottom surface of the boss flange portion, wherein a sealing groove facing a radially outer end of the sealing step is formed to be recessed upward below an inner end of the liner portion extending along the bottom surface of the boss flange portion; a sealing portion seal-coupled to a boundary area between a bottom of the boss flange portion and said top of the liner portion to block leakage of fluid accommodated in the accommodation space, wherein the sealing portion comprises a sealing plate and a sealing shape-matching portion integrally extending upward from a top of the sealing plate, said sealing plate surrounding a radial outside of the through hole along a circumferential direction and having an inner top surface arranged radially inward in radial direction and in surface contact with the bottom surface of the sealing step, and an outer top surface arranged radially outward in radial direction and in surface contact with a bottom surface of a liner extension portion formed above the liner portion, and said sealing shape-matching portion having a radially inner surface in surface contact with the outer end of the sealing step and a top surface pressed against and shape-matched with the sealing groove; characterized in that a first sealing protrusion and a third sealing protrusion extend upward from said top surfaces (31a, 31b) of the sealing plate, and a second sealing protrusion is formed on a top surface of the sealing shape-matching portion and integrally extends and protrudes toward the sealing groove along a circumferential direction so that a top end is elastically pressurized toward the sealing groove.