A clamping device including a mount table having an opening portion provided in a center thereof, a step portion provided at an edge of the opening portion and having a reference surface on which a workpiece is to be placed, the clamping device being configured to clamp the workpiece placed on the reference surface, the clamping device further comprises: a securing member having a disk-shaped bottom plate portion in which an opening portion is formed and which comes into surface contact with an end surface of the workpiece placed on the reference surface of the mount table, the opening portion being larger than an inner diameter of the workpiece and smaller than an outer diameter of the workpiece, and a cylindrical portion which has a cylindrical shape connected to an edge portion of the bottom plate portion characterized in that the cylindrical portion further includes a plurality of claw portions and a protrusion portion, the claw portions being provided at predetermined intervals in a circumferential direction and protruding toward a center, the protrusion portion being formed over an entire circumference of a base end portion of the cylindrical portion and protruding toward the center; and pressing-force applying means for applying a pressing force to the securing member toward the end surface of the workpiece, the pressing-force applying means having a cylinder shape surrounding the mount table and including a first outer protrusion portion protruding radially over the entire circumference of a portion below an outer circumferential potion of the pressing-force applying means and a plurality of second outer protrusion portions being formed at circumferential positions above the outer circumferential portion of the pressing-force applying means and protruding radially, the first outer protrusion portion and the second outer protrusion portions being capable of being coupled to the claw portions and the protrusion portion, the pressing-force applying means is attached to the mount table in such a manner as to be movable upward and downward, and a first hydraulic chamber and a second hydraulic chamber capable of supplying and discharging hydraulic oil are formed by a protrusion portion provided to an inner circumferential portion of the pressing-force applying means and by a groove portion which is formed at an outer circumferential portion of the mount table and into which the protrusion portion of the pressing-force applying means is fitted, supplying the hydraulic oil to the first hydraulic chamber causes the pressing-force applying means to bias the workpiece toward the reference surface of the mount table through the securing member and clamp the workpiece, and supplying the hydraulic oil to the second hydraulic chamber causes the pressing-force applying means to release pressing force of the workpiece toward the reference surface of the mount table through the securing member and unclamp the workpiece. a securing member having a disk-shaped bottom plate portion in which an opening portion is formed and which comes into surface contact with an end surface of the workpiece placed on the reference surface of the mount table, the opening portion being larger than an inner diameter of the workpiece and smaller than an outer diameter of the workpiece, and a cylindrical portion which has a cylindrical shape connected to an edge portion of the bottom plate portion characterized in that the cylindrical portion further includes a plurality of claw portions and a protrusion portion, the claw portions being provided at predetermined intervals in a circumferential direction and protruding toward a center, the protrusion portion being formed over an entire circumference of a base end portion of the cylindrical portion and protruding toward the center; and pressing-force applying means for applying a pressing force to the securing member toward the end surface of the workpiece, the pressing-force applying means having a cylinder shape surrounding the mount table and including a first outer protrusion portion protruding radially over the entire circumference of a portion below an outer circumferential potion of the pressing-force applying means and a plurality of second outer protrusion portions being formed at circumferential positions above the outer circumferential portion of the pressing-force applying means and protruding radially, the first outer protrusion portion and the second outer protrusion portions being capable of being coupled to the claw portions and the protrusion portion, the pressing-force applying means is attached to the mount table in such a manner as to be movable upward and downward, and a first hydraulic chamber and a second hydraulic chamber capable of supplying and discharging hydraulic oil are formed by a protrusion portion provided to an inner circumferential portion of the pressing-force applying means and by a groove portion which is formed at an outer circumferential portion of the mount table and into which the protrusion portion of the pressing-force applying means is fitted, supplying the hydraulic oil to the first hydraulic chamber causes the pressing-force applying means to bias the workpiece toward the reference surface of the mount table through the securing member and clamp the workpiece, and supplying the hydraulic oil to the second hydraulic chamber causes the pressing-force applying means to release pressing force of the workpiece toward the reference surface of the mount table through the securing member and unclamp the workpiece.