A clamp device including a detection mechanism provided to detect whether the clamp device is in a clamped state or an unclamped state. The detection mechanism includes a tilt plate tiltably placed inside a switch holder installed on a body and a sliding rod displaced together with a piston rod and displaced in the direction normal to the tilt plate. When the sliding rod presses and tilts the tilt plate while a piston is displaced, a second detection switch detects that the clamp device is in the unclamped state.