An introducer assembly (10, 200, 410, 610) comprising: an introducer sheath (11, 411, 611) which is provided with a sheath tube (20, 420, 620), a sheath hub (30, 430, 630) attached to the proximal end of the sheath tube (20, 420, 620), and a hemostatic valve having a passage for a catheter to pass through, the hemostatic valve being attached to the proximal end of the sheath hub (30, 430, 630); a dilator (13, 413, 613) which is provided with a dilator tube and a dilator hub (60, 460, 660) attached to the proximal end of the dilator tube; a cap which is provided with a through-hole for the dilator tube (50,) to pass through, with the cap having the dilator tube being passed through its through-hole and the cap being arranged between the sheath hub (30, 430, 630) and the dilator hub (60, 460, 660), and being capable of moving from a first position away from the sheath hub (30, 430, 630) to a second position at which the cap is engaged with the sheath hub (30, 430, 630); an engaging member which is formed on the sheath hub (30, 430, 630) and the cap and causes the cap to engage with the sheath hub (30, 430, 630), thereby holding the cap at the second position; and a deforming member which is arranged between the hemostatic valve and the cap and presses the hemostatic valve as the cap is moved to the second position, thereby applying a compressive force to the hemostatic valve in such a direction as to close the passage, the introducer sheath and the dilator (13, 413, 613) being previously integrated with each other while the dilator tube passes through the through-hole of the cap and the passage of the hemostatic valve and the cap is held at the first position, characterized in that the hemostatic valve has an inclined part which projectingly extends aslant toward the cap, such that the inclined part is tapered towards the cap, resulting in a trapezoidal cross-section of the hemostatic valve. an introducer sheath (11, 411, 611) which is provided with a sheath tube (20, 420, 620), a sheath hub (30, 430, 630) attached to the proximal end of the sheath tube (20, 420, 620), and a hemostatic valve having a passage for a catheter to pass through, the hemostatic valve being attached to the proximal end of the sheath hub (30, 430, 630); a dilator (13, 413, 613) which is provided with a dilator tube and a dilator hub (60, 460, 660) attached to the proximal end of the dilator tube; a cap which is provided with a through-hole for the dilator tube (50,) to pass through, with the cap having the dilator tube being passed through its through-hole and the cap being arranged between the sheath hub (30, 430, 630) and the dilator hub (60, 460, 660), and being capable of moving from a first position away from the sheath hub (30, 430, 630) to a second position at which the cap is engaged with the sheath hub (30, 430, 630); an engaging member which is formed on the sheath hub (30, 430, 630) and the cap and causes the cap to engage with the sheath hub (30, 430, 630), thereby holding the cap at the second position; and a deforming member which is arranged between the hemostatic valve and the cap and presses the hemostatic valve as the cap is moved to the second position, thereby applying a compressive force to the hemostatic valve in such a direction as to close the passage, the introducer sheath and the dilator (13, 413, 613) being previously integrated with each other while the dilator tube passes through the through-hole of the cap and the passage of the hemostatic valve and the cap is held at the first position, characterized in that the hemostatic valve has an inclined part which projectingly extends aslant toward the cap, such that the inclined part is tapered towards the cap, resulting in a trapezoidal cross-section of the hemostatic valve.