A clamp apparatus for clamping a workpiece with a pair of clamp arms by rotation of the clamp arms, comprising: a body; a drive unit disposed on the body and having a displaceable body (76, 78) which is displaced in axial directions; first and second clamp arms (14, 16), which are supported rotatably with respect to the body and are arranged in a mutually confronting manner; a driving force transmission mechanism connected to the drive unit, and which converts linear displacement along an axial direction of the drive unit into rotary displacement and transmits a driving force of the drive unit to the first and second clamp arms (14, 16); and positioning means connected to the driving force transmission mechanism for positioning and retaining the workpiece in a predetermined position at a stage prior to clamping the workpiece, by restricting rotation of the second clamp arm in advance of the first clamp arm when clamping of the workpiece is carried out, characterized in that the positioning means comprises: a cam groove formed in the second clam arm; and a roller supported rotatably on one end of a block that is connected to the displaceable body, the roller being movable along the cam groove; and the cam groove comprising: a rotation groove part, which is inclined with respect to a displace-ment direction of the displaceable body, and is engaged with the roller when the second clamp arm is rotated; and a restriction groove part connected to the rotation groove part, and which extends along the displacement direction of the displaceable body, and is engaged with the roller when rotation of the second clamp arm is restricted, and that the driving force transmission mechanism comprises a link pin inserted through the other end of the block, and a link arm supported for rotation between the link pin and a second arm pin of the first clamp arm. a body; a drive unit disposed on the body and having a displaceable body (76, 78) which is displaced in axial directions; first and second clamp arms (14, 16), which are supported rotatably with respect to the body and are arranged in a mutually confronting manner; a driving force transmission mechanism connected to the drive unit, and which converts linear displacement along an axial direction of the drive unit into rotary displacement and transmits a driving force of the drive unit to the first and second clamp arms (14, 16); and positioning means connected to the driving force transmission mechanism for positioning and retaining the workpiece in a predetermined position at a stage prior to clamping the workpiece, by restricting rotation of the second clamp arm in advance of the first clamp arm when clamping of the workpiece is carried out, characterized in a cam groove formed in the second clam arm; and a roller supported rotatably on one end of a block that is connected to the displaceable body, the roller being movable along the cam groove; and the cam groove comprising: a rotation groove part, which is inclined with respect to a displace-ment direction of the displaceable body, and is engaged with the roller when the second clamp arm is rotated; and a restriction groove part connected to the rotation groove part, and which extends along the displacement direction of the displaceable body, and is engaged with the roller when rotation of the second clamp arm is restricted, and that the driving force transmission mechanism comprises a link pin inserted through the other end of the block, and a link arm supported for rotation between the link pin and a second arm pin of the first clamp arm. a rotation groove part, which is inclined with respect to a displace-ment direction of the displaceable body, and is engaged with the roller when the second clamp arm is rotated; and a restriction groove part connected to the rotation groove part, and which extends along the displacement direction of the displaceable body, and is engaged with the roller when rotation of the second clamp arm is restricted, and