Abstract: A distal end of an inserted pole makes contact with a bottom surface or tapering surface of a portion corresponding to a bottom of an insertion hole. A lever rotates about a pivot shaft, and a pressing surface facing from a recessed portion formed in the insertion hole presses a side surface portion of the pole. The pole pressed by the pressing surface is mainly received by ridge portions, particularly the ridge portions positioned on the opposite side with a central axis therebetween in relation to the pressing surface. The ridge portions are positioned on both sides of the pole with the cross section along the line A-A therebetween as viewed in the axial direction. As the lever is rotatably operated in the tightening direction, the pressing position gradually approaches the central axis, thereby increasing the pressing force.

Abstract: A distal end of an inserted pole makes contact with a bottom surface or tapering surface of a portion corresponding to a bottom of an insertion hole. A lever rotates about a pivot shaft, and a pressing surface facing from a recessed portion formed in the insertion hole presses a side surface portion of the pole. The pole pressed by the pressing surface is mainly received by ridge portions, particularly the ridge portions positioned on the opposite side with a central axis therebetween in relation to the pressing surface. The ridge portions are positioned on both sides of the pole with the cross section along the line A-A therebetween as viewed in the axial direction. As the lever is rotatably operated in the tightening direction, the pressing position gradually approaches the central axis, thereby increasing the pressing force.