A dust cover for a ball joint, the 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 in the other end of said ball stud , the shaft being fastened and fixed to a knuckle ; the dust cover comprising: a one end large-diameter opening portion having an embedded reinforcing ring , the one end large-diameter opening portion being fixable and retainable to an outer peripheral surface of said socket ; the other end small-diameter opening portion having an embedded reinforcing ring , the other end small-diameter opening portion being retainable to said shaft ; a film portion connecting said large-diameter opening portion and said small-diameter opening portion ; and the dust cover being made of an elastic material, characterized in that in an unmounted state, a maximum inner diameter (X) of said film portion is larger than an inner diameter (Y) of said reinforcing ring which is embedded in said one end large-diameter opening portion , said film portion is formed into a drum shape, and a thickness (t1) of a maximum outer diameter (Z) portion of said film portion is formed to be thinner than a thickness (t2) in an other end small-diameter opening portion side of said film portion and that a thickness (t3) in an one end large-diameter opening portion side of said film portion . a spherical head portion formed in one end of a ball stud , the spherical head portion being retained within a socket ; a shaft in the other end of said ball stud , the shaft being fastened and fixed to a knuckle ; the dust cover comprising: a one end large-diameter opening portion having an embedded reinforcing ring , the one end large-diameter opening portion being fixable and retainable to an outer peripheral surface of said socket ; the other end small-diameter opening portion having an embedded reinforcing ring , the other end small-diameter opening portion being retainable to said shaft ; a film portion connecting said large-diameter opening portion and said small-diameter opening portion ; and the dust cover being made of an elastic material, a one end large-diameter opening portion having an embedded reinforcing ring , the one end large-diameter opening portion being fixable and retainable to an outer peripheral surface of said socket ; the other end small-diameter opening portion having an embedded reinforcing ring , the other end small-diameter opening portion being retainable to said shaft ; a film portion connecting said large-diameter opening portion and said small-diameter opening portion ; and the dust cover being made of an elastic material, characterized in that in an unmounted state, a maximum inner diameter (X) of said film portion is larger than an inner diameter (Y) of said reinforcing ring which is embedded in said one end large-diameter opening portion , said film portion is formed into a drum shape, and a thickness (t1) of a maximum outer diameter (Z) portion of said film portion is formed to be thinner than a thickness (t2) in an other end small-diameter opening portion side of said film portion and that a thickness (t3) in an one end large-diameter opening portion side of said film portion . <EOS>