a description will be given below of an embodiment in accordance with the present invention with reference to the accompanying drawings . [ 0033 ] fig1 shows a ball joint 1 used in a connection portion of a stabilizer for a vehicle . the ball joint 1 in accordance with the embodiment is provided with a ball stud 20 having a spherical head portion 20 a in an end portion , a ball seat 30 to which the spherical head portion 20 a is slidably fitted , and a housing 40 to which the ball seat 30 is pressure inserted so as to receive the ball seat 30 . a rivet - like engagement portion 30 b formed in a bottom portion of the ball seat 30 extends through a bottom portion of the housing 40 so as to engage therewith , whereby the ball seat 30 is clamped to the housing 40 , so that it is possible to prevent the ball seat 30 from rotating around an axis and coming off from the housing 40 . further , one end of a support bar 70 is firmly fixed to the housing 40 . the ball stud 20 pivots in the ball seat 30 in a universal manner around the spherical head portion 20 a , that is , in such a manner as to freely tilt and rotate around the axis . fig2 is a view showing the ball joint 1 , in accordance with the embodiment , in an enlarged manner . as shown in fig2 a , a shaft portion 20 b connected to the spherical head portion 20 a of the ball stud 20 is formed in a tapered shape tapered toward the spherical head portion 20 a . further , a front end portion of the spherical head portion 20 a has a large portion cut off , and a flat portion 20 c is formed thereat . a distance from a center o of the spherical head portion 20 a to the flat portion 20 c is set to be 0 . 25 to 0 . 4 times the diameter of the spherical head portion 20 a . the ball seat 30 is formed in a closed - end cylindrical shape having a flange portion 31 in an upper end edge , and a spherical seat 32 to which the spherical head portion 20 a is fitted is formed in an inner portion thereof . as shown in fig2 b , a convex portion 33 which is thicker than a peripheral portion is formed in a center of a bottom portion of the ball seat 30 . the ball seat 30 is formed , for example , by a resin such as a polyacetal , a polybutylene terephthalate or the like . a bevel - like dust cover 60 is provided between the flange portion 31 of the ball seat 30 and a flange portion 21 formed in the ball stud 20 . the housing 40 is structured such that a flange portion 42 is integrally formed in an upper end edge of a closed - end cylindrical main body portion 41 , and an inner portion thereof is formed as a receiving portion for the ball seat 30 . a thickness of the flange portion 42 is made thinner than a thickness of the main body portion 41 . further , an outer radius of curvature of a cross section in a crossing portion between the main body portion 41 and the flange portion 42 is made smaller than the thickness of the main body portion 41 . next , a description will be given of a method of manufacturing the housing 40 with reference to fig3 . for manufacturing the housing 40 , a transfer metal mold in which a plurality of working stages are provided in one metal mold is preferably employed . first , as shown in fig3 a , a formed body 45 having the shape of the housing is draw molded from a steel plate . next , a hole 40 a to which the engagement portion 30 b of the ball seat 30 is inserted is formed in the next working stage . in this state , the formed body 45 has a uniform thickness t 1 . next , the formed body 45 is set in a lower mold ( not shown ) of the next stage so as to clamp an outer periphery , and a punch is inserted to an opening portion of the formed body 45 . accordingly , a thickness near the opening portion moves to a side of the opening portion , whereby the thickness is reduced . therefore , as shown in fig3 b , a portion 45 a having the thickness t 1 of the raw material and a portion 45 b having a thickness t 2 thinner than the thickness t 1 are formed in a tubular portion of the formed body 45 , and these two portions 45 a and 45 b are smoothly connected by curved surfaces having radii r 1 and r 2 of curvature . in this embodiment , the thickness t 2 is set to be about 60 % of the thickness t 1 , whereby it is possible to make the radius r 2 of curvature smaller than the thickness t 1 . next , the formed product 45 is transferred to the next stage , and as shown in fig3 c , is expanded by the portion having the radius r 2 of curvature , whereby the housing 40 constituted by the main body portion 41 and the flange portion 42 is formed . in this case , in the crossing portion between the main body portion 41 and the flange portion 42 , the radius r 2 of curvature formed by the thickness reduction mentioned above is left as it is . accordingly , an outer radius of curvature of the crossing portion is smaller than the thickness t 1 of the raw material . in the ball joint 1 having the structure mentioned above , since the flange portion 42 is compression molded so as to be thinner than the thickness of the main body portion 41 , it is possible to reduce a size h from a center line 70 a of the support bar 70 to an end surface of the flange portion 42 . accordingly , it is possible to increase a tensile strength and a buckling strength of the support bar 70 . in particular , in the embodiment mentioned above , since the outer radius r 2 of curvature in the cross section of the crossing portion between the flange portion 42 and the main body portion 41 is made smaller than the thickness t 1 , the distance l 1 of the linear portion of the housing inner peripheral surface is long , whereby it is possible to increase the drawing strength of the ball stud 20 . further , in the embodiment mentioned above , since it is possible to form the structure to be as thin as possible while preventing the bottom portion from being cracked , by cutting off a large part of the front end portion of the spherical head portion 20 a so as to form the flat portion , and forming the convex portion 33 in the center of the bottom portion in the ball seat 30 , it is possible to arrange the ball stud 20 significantly closer to the bottom portion side . as a result , as shown in fig2 a , the center o of the spherical head portion 20 a is arranged on the center line 70 a of the support bar 70 , and it is possible to substantially completely prevent the bending moment from being generated in the support bar 70 . further , in the embodiment mentioned above , since the shaft portion 20 b of the ball stud 20 is formed in the tapered shape , there is an advantage that a wide angle of swing can be secured .