The servo device has a cylinder (10) with an open end, a first piston (12) sliding in the cylinder and a rod connected to the first piston. A cover part (19) closes the open end of the cylinder. The cover part, the cylinder and the first piston form a first oil chamber (Ra) in the cylinder. Oil pressure to the first oil chamber is fed through an aperture in the cover part. The open end of the cylinder touches a control box (2) of the automatic gear, and is connected to an oil chamber (21) in the control box. A sealing part (3) is arranged between the cover part and the control box to cover an opening of the cover part and an opening of the oil channel of the control box. A device (18) for regulating axial movement prevents loosening of the cover part from the open end of the cylinder.
