An arm structure includes: a pair of arms a support post a support member a cam provided in one of the pair of arms a shaft member inserted through a lateral insertion hole of the support member and a pair of ring-shaped members provided in the end portions of the shaft member, wherein by turning the one of the arms around its axis thereby to make the cam revolve, a force works to make the shaft member move and due to its reaction force the ring-shaped members pressure-contact the support member and a slit portion is narrowed. As a result, a longitudinal insertion hole is diameter-reduced and becomes in a state of pressure-contacting the support post, leading to restriction of rotation of the arm around the support post, and the cam pressure contacts the one of the arms, leading to restriction of sliding of the arm in a long side direction, and the ring-shaped members pressure-contact the support member and leading to restriction of rotation of the arm by the shaft member.
