Disclosed is a clamp apparatus comprising a cylinder section for displacing a rod member provided at the inside of a body in an axial direction; a toggle link mechanism for converting rectilinear motion of the rod member into rotary motion; an arm for making rotation by a predetermined angle in accordance with a driving action of the cylinder section; and a lock mechanism for maintaining a clamped state of a workpiece effected by the arm even when transmission of driving force of the cylinder section to the arm is stopped.