To respectively independently fix a plurality of extendible and contractible leg parts.

The fixing structure of the leg parts comprises: a rod-like first leg part 220 in a roughly oval shape in the plane view for which two or more stages of engaging grooves 222 are formed at the circular arcuate part 221 of a peripheral surface; a second leg part 230 having a roughly cylindrical shape, which is provided with a pair of engaging pieces 231 facing each other to be engaged with the engaging groove 222 at the upper end; and a fastening implement 240 vertically movably fitted to the first leg part 220. The fastening implement 240 is provided with a turning preventing projection 243 inserted into a gap G between the first leg part 220 and the second leg part 230 in a state that the engaging pieces 231 of the second leg part 230 are engaged with the engaging groove 222 of the first leg part 220.

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