A nipple assembly includes nipple bodies configured so that one end portion of each of the nipple bodies is inserted into a housing so as to be coupled to a cooling medium inlet or a cooling medium outlet of a heat exchanger and a remaining end portion of each of the nipple bodies protrudes outwards from the housing, at least one first sealing unit sealing a gap between the one end portion of each of the nipple bodies and the cooling medium inlet or the cooling medium outlet, a flange portion formed at a position between two end portions of each of the nipple bodies to be coupled to the housing, and a second sealing unit including a first sealing portion sealing a gap between the flange portion and the housing and a second sealing portion configured to extend from the first sealing portion.