A clamping device in which there is no need to carry out troublesome setting operation of a clamping position according to a size of a workpiece and wear of respective components and an operation efficiency can be further improved. A worm which is engaged with a worm wheel provided to a periphery of a rotating shaft of a clamping arm and which is connected to an electric motor can reciprocate in an axial direction. On the axis, a clamping force applying mechanism to be actuated in response to contact of the rotated clamping arm with the workpiece to apply an axial force in a direction of the axis to the worm is provided. The clamping arm is rotated by driving the worm for rotation by the electric motor and the clamping force applying mechanism applies a clamping force to the clamping arm which has come in contact with the workpiece.