A pressure vessel comprising: a boss tail portion comprising a boss extension portion having a cylindrical shape and a boss flange portion integrally expanding outward in a radial direction along a circumferential direction above the boss extension portion; a liner portion having a container shape in which an accommodation space is formed to accommodate a fluid therein and a bottom is sealed and coupled, through integral insertion-injection molding, along a top surface of the boss flange portion; a composite cover portion provided to surround an outer surface of the liner portion and to have a bottom end sealed and coupled while surrounding a bottom surface of the boss flange portion and an outer surface of the boss extension portion wherein the boss flange portion comprises a recessed groove recessed downward from a top surface and a protruding edge portion extending and protruding upward while surrounding a periphery of the recessed groove to thereby define a blind hole, wherein a liner extension portion extending radially inward along the top surface of the boss flange portion and having a bottom surface pressed against the top surface of the boss flange portion is primarily formed on the liner portion, and wherein a sealing cover portion extending further from an inner end of the liner extension portion toward an outline of an inner surface corresponding to an outline of an outer surface of the protruding edge portion is secondarily formed, characterized in that the boss flange portion has a shape-matching and pressing groove surrounding a radial outside of the protruding edge portion and having a cross section continuously increasing inwardly in width, and liner extension portion has a shape-matching extension portion having a cross section gradually increasing in width toward its end to be pressed against and shape-matched with the shape-matching and pressing groove, the boss flange portion comprises a sealing step formed to protrude from a top of the shape-matching and pressing groove radially outward along a circumferential direction, and the inner end of the liner extension portion is pressed against and shape-matched with an outer surface of the sealing step. a boss tail portion comprising a boss extension portion having a cylindrical shape and a boss flange portion integrally expanding outward in a radial direction along a circumferential direction above the boss extension portion; a liner portion having a container shape in which an accommodation space is formed to accommodate a fluid therein and a bottom is sealed and coupled, through integral insertion-injection molding, along a top surface of the boss flange portion; a composite cover portion provided to surround an outer surface of the liner portion and to have a bottom end sealed and coupled while surrounding a bottom surface of the boss flange portion and an outer surface of the boss extension portion wherein the boss flange portion comprises a recessed groove recessed downward from a top surface and a protruding edge portion extending and protruding upward while surrounding a periphery of the recessed groove to thereby define a blind hole, wherein a liner extension portion extending radially inward along the top surface of the boss flange portion and having a bottom surface pressed against the top surface of the boss flange portion is primarily formed on the liner portion, and wherein a sealing cover portion extending further from an inner end of the liner extension portion toward an outline of an inner surface corresponding to an outline of an outer surface of the protruding edge portion is secondarily formed, characterized in that the boss flange portion has a shape-matching and pressing groove surrounding a radial outside of the protruding edge portion and having a cross section continuously increasing inwardly in width, and liner extension portion has a shape-matching extension portion having a cross section gradually increasing in width toward its end to be pressed against and shape-matched with the shape-matching and pressing groove, the boss flange portion comprises a sealing step formed to protrude from a top of the shape-matching and pressing groove radially outward along a circumferential direction, and the inner end of the liner extension portion is pressed against and shape-matched with an outer surface of the sealing step.