A clamp apparatus includes a clamp unit capable of retaining a workpiece, a cylinder driven under the supply of a pressure fluid, and a drive force transmission mechanism, which is capable of transmitting a drive force from the cylinder to the clamp unit. The drive force transmission mechanism includes a guide body disposed displaceably inside of a body, two pairs of rollers retained rotatably on both side surfaces of the guide body, and a spindle inserted through a second link groove of the guide body that rotatably supports a clamp arm. The clamp arm is rotatably operated and is capable of clamping the workpiece under a displacement action of the guide body.