To reliably engage a locking part and a locking object part with each other by slidingly contacting a slidingly movable part with a slidingly contacting surface by rotary operation of a fastening bolt member, slidingly moving a first hook member in the upward direction to a leg member, and simultaneously pulling a second hook member in the upward direction by the first hook member.

A leg piece 26 is inserted into a dovetail groove 38 of a rail member 6, and is received by this, and a second hook member 5 in a back to back condition is opposed to a space. A screw bolt 35 enters a knob handle 36 by rotary operation of the knob handle 36, and the knob handle 36 presses a first hook member 3 to the leg member 2 side, and while a slidingly movable part 31 is slidingly contacting with a slidingly contacting surface 28, the first hook member 3 slidingly moves in the upward direction. A downward bending member 32 of the first hook member 3 pulls the second hook member 5 in the upward direction by sliding motion of this first hook member 3 while engaging with a locking hole 44 of the second hook member 5. Therefore, the second hook member 5 is reliably engaged with the dovetail groove 38 of the rail member 6.