A clamped object is reliably clamped. A clamp sensor includes a pair of clamp arms that are formed so as to be substantially arc-shaped in plan view, that are configured so that at least one of the clamp arms is rotatable so that respective front ends of the clamp arms open and close, and that form a ring-shaped body in a state where the front ends are closed. Front end portions of the clamp arms have a pair of facing surfaces that construct the outer circumferential surface and the inner circumferential surface of the ring-shaped body, a pair of facing surfaces that construct two side surfaces of the ring-shaped body, a pair of facing surfaces that are inclined to the facing surfaces, and a pair of facing surfaces that are inclined to the facing surfaces.