A clamping device is capable of engaging a clamping object with an inner wall of hole or side wall of the clamping object.The clamping device 1 is provided with a rod support for moving an engagement portion 25 of a clamping rod 3 in a direction roughly rectangular to the longitudinal direction of the clamping rod 3 and switchably rotatably supports the clamping rod 3 in a clamping main body across a clamping position and a clamp release position, a piston drive mechanism 6 for driving a piston 4 across the clamping position and the clamp release position, and a cam mechanism 7 for driving the engagement portion 25 in the clamping direction by the driving force of piston drive mechanism 6 for driving the piston 4 to the clamping position.