To provide a chair with a tilting seating surface which periodically tilts a seat in a predetermined direction at a fixed position of a floor surface.

The chair with a tilting seating surface includes: a leg having a leg post member; and a tilting unit provided at an upper end of the leg post member and periodically tilting the seat from a base position in the predetermined direction. The leg includes: a plurality of leg blades protruded outward in a radial direction from a base end of the leg post member and mounted with casters at ends thereof; a plurality of stopper members each provided in at least one leg blade and the other leg blades separated from the one leg blade at 90 degree or more with centering the leg post member in the plurality of leg blades and configured to be capable of being grounded in positions radially outer than those of tip ends of the plurality of leg blades; and a locking device for locking the plurality of stopper members by pulling the members radially inward. The plurality of stopper members are grounded in the positions radially outer than those of the tip ends of the plurality of leg blades when the locking caused by the locking device is unlocked.