plate portion 17 and the third plate portion 18 form an accommodating space 19 which can accommodate therein the split sleeve 3 the split sleeve 3 is accommodated in the accommodating space 19 through open end portions 20 the first plate portion 16 includes a first flat surface 21 which extends parallel to an x axis the first flat surface 21 contacts with a peripheral surface of the split sleeve 3 when the split sleeve 3 is accommodated in the accommodating space 19 the second plate portion 17 includes a second flat surface 22 which is formed to stand vertically on one side end 16 a of the first plate portion 16 the second flat surface 22 is configured to contact with the circumferential surface of the split sleeve 3 when the split sleeve 3 is accommodated in the accommodating space 19 the third plate portion 18 stands on the other side end 16 b of the first plate portion 16 to project towards the second plate portion 17 the third plate portion 18 includes a curved surface 23 which extends parallel to the x axis the curved surface 23 projects towards the second plate portion 17 and contacts with the peripheral surface of the split sleeve 3 when the split sleeve 3 is accommodated within the accommodating space 19 the operation and function of the holding portion 13 when the split sleeve 3 is accommodated in the accommodating space 19 through the open end portions 20 will be described when the split sleeve 3 is accommodated within the accommodating space 19 the first flat surface 21 and the second flat surface 22 function as.