A chair (1) includes an installation base (10), a leg column (11) supported by the installation base (10) in a tiltable manner, a seat body (12) supported on an upper part of the leg column (11), an elastic member (19) which urges the leg column (11) to an initial tilted posture, and a coming-off restriction member (22) which restricts coming-off of the leg column (11) from the installation base (10). On a support wall (16) of the installation base (10) having an insertion hole (20) through which part of the coming-off restriction member (22) is inserted, a slide guide (21) coming into slidable contact with a slide guide (21) at a peripheral edge portion of a lower surface side of the insertion hole (20) is provided. The coming-off restriction member (22) has an upwardly protruding spherical guide surface (23) which is slidably brought into contact with the slide guide (21) below the insertion hole (20).