PURPOSE: To easily, quickly and surely shift from the unlock state to the lock state and from the lock state to the unlock state of a stably supported movable member for which rotation adjustment is possible by simple operations.

CONSTITUTION: This mechanism is provided with a connection member 7 engaged with a projecting engaging part 20 of a supporting base 5 passing through the oblong hole 11 of the movable member 3 for making the rotation adjustment in plural directions possible and connecting the movable member 3 to the supporting base 5 and a rotation control member 10 engaged with the projecting engaging part 20 of the supporting base 5 passing through the oblong hole 11 from the opposite direction to the connection member 7 for selectively taking an engaging position for releasing the rotation regulation of the movable member 3 and the engaging position for blocking rotation of the movable member 3 by moving in the direction along the center axis line of the projecting engaging part 20.