To perform the assembling and folding of a movable leg by simple operation.

The insertion hole 24, in which the connection shaft 23 of the movable leg 20 is held, of a bracket 22 is formed as a slot to make it possible to slide the movable leg 20 from a downward drawing-out position to the push-in position toward the bracket 22 while a notch 25 is provided to the bracket 22 and the stopper pin 26 fitted in the notch 25 at the push-in position is provided to the movable leg 20 to regulate the rotation at the push-in position of the movable leg 20 while permitting the rotation of the movable leg 20 at the drawing-out position. A lock spring 28, which is locked with the recessed part 34 provided to the bracket 22 at the push-in position to regulate the slide of the movable leg 20, is provided to the movable leg 20 to enable the slide of the movable leg 20 to the drawing-out position when the lock spring 28 is separated from the recessed part 34.