To easily position and firmly attach a long pipe-like crossbar to left and right brackets.

A circular arc-shaped receiving part 31a for receiving/supporting the lower surface of the crossbar (backrest bar) 30 is disposed on the front end of each bracket 31, and a projecting shaft 33 facing upward is disposed on top of the receiving part 31a. The projecting shaft 33 is engaged with a through hole 37 bored on the lower surface of the crossbar 30, and the lower surface of the crossbar 30 is supported by the receiving part 31a. The crossbar 30 is fixed to the bracket 31 by screw-engaging a screw inserted from under an attaching hole 34 in the vertical direction formed in the receiving part 31a with a screw hole formed on the lower surface of the crossbar 30.