A clamp apparatus includes a pair of first and second clamp arms supported rotatably on a body. The clamp apparatus also includes a manual release mechanism capable of manually releasing a clamped state in the case that an output from a drive unit to the first and second clamp arms is stopped and the clamped state of a workpiece is locked. The manual release mechanism includes a release lever disposed rotatably on a first plate body. By an operator operating the release lever to thereby rotate the release lever, a block body of a driving force transmission mechanism is pressed and moved upwardly, and pressing forces applied to the first and second clamp arms in widthwise outside directions thereof by a pair of rollers, are released. Consequently, gripping members of the first and second clamp arms are rotated in directions away from each other to thereby cause an unclamped state.