PURPOSE: To apply relative displacement to both sole plates, by inserting the screw section of a regulating rod through the hole of an engaging member on one side, to be screwed into the screw hole of an engaging member on the other side, and by butting the tip of a main large diameter section to the peripheral wall of the hole of the engaging member on one side.

CONSTITUTION: To a sole plate 11a on one side, an engaging member 14 having a screw hole 14b is fitted. To a sole plate 11b on the other side, an engaging member 16 having an inserting hole 16b is fitted. Then, the screw section 17a of a regulating rod 17 is inserted through the inserting hole 16b and is screwed into the screw hole 14b and is butted to a peripheral wall at the tip of the main large diameter section 17b. Accordingly, by rotating the rod 17, a distance between the engaging members 14, 16 can be changed, and so both the sole plates 11a, 11b can be relatively displaced.