To enable a user to positionally adjust the boots of both feet along both of the lengthwise and breadthwise directions of a skid board by applying the constitution that the positioning means is formed to be movable in all directions approximately in parallel with the skid board and at the same time, rotatable at any angle around an approximately vertical axis.

A screw 6 is tightened to a center disc 4, and the center disc 4 itself is secured to a skid board 2. As a result, the screw 6 keeps a base plate 3 in pressed contact with the skid board 2 and the center disc 4 via a washer 5 laid between the head and the bottom of the counterbore 48 of the base plate 3. Then, the base plate 3 is fixed to the skid board 2 via friction on the surface 44 of the skid board 2 and or the center disc 4. Thus, when the screw 6 is loosened, the base plate 3 can be moved along a restricted gap within a plane approximately parallel to the skid board 2 and at the same time, can be rotated at any angle around an approximately vertical axis.