PURPOSE: To enable pre-assembling and automate fitting by forming a means for clamping parallel walls of a case in which the inner surface of the head is inserted by both wings of a rider.

CONSTITUTION: This device incldes a head 3 having an inner surface 4 inserted in a case 2 including two parallel walls 5, 5A. One or plural cases 2 are assembled by the head 3 and a rider 8 U-shaped in section including two parallel wing parts 10, 11 and a cross wall 14 integrally formed with the inner surface 4 of the head 3. Further, a means for clamping parallel walls 5, 5A of one or plural cases 2 is formed by both parallel wing parts 10, 11 of the rider 8. Thus, before the case 2 is assembled to the inner surface 4 of the head 3, pre- assembling including a control mechanism can be performed.