A clamping mechanism for a semiconductor substrate includes: a C-shaped pickup plate; a susceptor top plate having a periphery adapted to receive and support an inner periphery portion of the C-shaped pickup plate thereon; and a clamp comprising (i) a top ring portion for clamping the substrate by sandwiching a periphery of the substrate between the top ring portion and the susceptor top plate and (ii) a pickup plate supporting portion adapted to support an outer periphery portion of the C-shaped pickup plate, wherein the C-shaped pickup plate is movable between the top ring portion and the pickup plate supporting portion, and the clamp is movable upward together with the C-shaped pickup plate and the susceptor top plate.