A dust cover for a ball joint comprising: a spherical head portion formed in one end of a ball stud, the spherical head portion being retained within a socket; a shaft provided in the other end of said ball stud, the shaft being fastened and fixed to a knuckle; a one end large-diameter opening portion fixed and retained to an outer peripheral surface of said socket; the other end small-diameter portion having a rigid ring integrally formed therein, the other end small-diameter portion being retained to said shaft; a film portion connecting said one end large-diameter opening portion and said other end small-diameter opening portion; and the dust cover for the ball joint being made of a rubber-like elastic material, wherein the dust cover for the ball joint comprises a retainer having a cylinder portion which is fitted and fixed to an outer peripheral surface of said shaft and is provided with an outer peripheral surface facing to an inner peripheral surface of said rigid ring, and a first flange portion which is provided with an upper end surface facing to a first end surface in said socket side of said rigid ring, characterized in that said rigid ring approximately has a rectangular shape in its cross section, and is structured such that one end of said film portion is connected to an outer peripheral surface side of the rigid ring, and the rigid ring is exposed at least in said inner peripheral surface, and said first end surface, and said retainer is constructed by integrally bonding a first gasket coming into contact with said inner peripheral surface in a sealing manner and made of a rubber-like elastic material to said outer peripheral surface, and integrally bonding a second gasket coming into contact with said first end surface in a sealing manner and made of a rubber-like elastic material to said upper end surface. a spherical head portion formed in one end of a ball stud, the spherical head portion being retained within a socket; a shaft provided in the other end of said ball stud, the shaft being fastened and fixed to a knuckle; a one end large-diameter opening portion fixed and retained to an outer peripheral surface of said socket; the other end small-diameter portion having a rigid ring integrally formed therein, the other end small-diameter portion being retained to said shaft; a film portion connecting said one end large-diameter opening portion and said other end small-diameter opening portion; and the dust cover for the ball joint being made of a rubber-like elastic material, wherein the dust cover for the ball joint comprises a retainer having a cylinder portion which is fitted and fixed to an outer peripheral surface of said shaft and is provided with an outer peripheral surface facing to an inner peripheral surface of said rigid ring, and a first flange portion which is provided with an upper end surface facing to a first end surface in said socket side of said rigid ring, characterized in that said rigid ring approximately has a rectangular shape in its cross section, and is structured such that one end of said film portion is connected to an outer peripheral surface side of the rigid ring, and the rigid ring is exposed at least in said inner peripheral surface, and said first end surface, and said retainer is constructed by integrally bonding a first gasket coming into contact with said inner peripheral surface in a sealing manner and made of a rubber-like elastic material to said outer peripheral surface, and integrally bonding a second gasket coming into contact with said first end surface in a sealing manner and made of a rubber-like elastic material to said upper end surface.