To solve a problem of the fact that unevenness in operation torque of a ball stud due to unevenness in size precision of each part occurs by a method wherein a ball sheet made of synthetic resin in which the ball part of a ball stud is fitted in a bracing state is pressed in a socket, the peripheral part of the opening part of a socket is securely caulked inward.

A ball joint assembly is set on a ball joint support bed 9. Operation torque of a ball stud 1 is detected through operation of an operation torque detecting device 8 comprising an oscillation rod 81, an operation rod 82, a drive part 83, and a strain gauge 84 as the peripheral part of the opening part 3a of a socket 3 is caulked by a caulking device comprising a caulking roller bed 6 having a caulking roller 7 and a press bed 5 to press downward the caulking roller bed 6. At a point of time when a detecting operation torque value attains a set proper value, caulking processing is completed and a ball joint freed from unevenness in operation torque is provided.