A base member includes a first through-hole. A first nut is fixed to an upper portion of the base member and includes a first screw hole that is in communication with the first through-hole. A coupling member is coupled to the base member in a state where the first nut is sandwiched between the coupling member and the base member and includes a second through-hole that is in communication with the first screw hole. A bolt portion of a support leg member is fit into the first screw hole while being inserted into the first through-hole, the first screw hole, and the second through-hole. A second nut is fastened to a portion of the bolt portion that protrudes upwardly from the second through-hole, to fix the bolt portion to the coupling member as well as sandwich the coupling member between the second nut and the first nut.