PURPOSE: To tighten the first shell and the second shell of servo-device without any chattering play by forming a rugged surface on either one of an outer peripheral part and a step part so as to press both of them to each other to produce elastic deformation so that the rugged surface may be formed in its flat state.

CONSTITUTION: The outer peripheral part 2c of a servo-device is placed on a lower mold 8, while an expanded part 2e formed on an outer peripheral part 2c is guided to match the position of a claw 11 under such a condition that the outer peripheral part 2c of a rear shell 2 is stored in the inside of the cylindrical part 1c of a front shell 1. Then, the step part 1b of the front shell 1 and the outer peripheral part 2c of the rear shell 2 are pressed to each other by means of an upper mold 10 and the lower mold 8. When the claw 11 is advanced, the claw 11 pushes a part of the cylindrical part 1c of the front shell into its inside so as to form a locking part 1d projecting radically inward. When the press-pushing force is released, the expanded part 2e and a plain part 2d are brought in elastic contact with the locking part 1d and step part 1b respectively so that the outer peripheral part 2c of the rear shell 2 is connected between the step part 1b of the front shell 1 and the locking part 1d without any chattering play.