In the disclosed clamp device, a lock chamber (16) and a release chamber (17) through which compressed air is supplied and removed are provided above and below a piston (15) inserted in a housing (5). A tapered sleeve (28), which can expand and contract in diameter, is mounted between a ring-shaped plug section (26) that protrudes upwards from the housing (15) and a taper hole (11) provided in a work pallet (2). An output rod (35) inserted into a cylindrical hole (26a) in the plug section (26) is coupled to the piston (15). Lock springs (49) comprising compressed coil springs are inserted into a plurality of spring-attachment holes (48) formed, in a concave shape, in the top wall (16a) of the lock chamber (16). The spring-attachment holes (48) are connected to the outer surface of the housing (15) via a through-hole (55) into which a pushing member (56) is inserted so as to form a seal. The lock springs (49) bias the sleeve (28) upwards with a prescribed force, via the pushing member (56).