A clamp sensor detects a detected value for a clamped object and includes: a main body with two main surfaces and two side surfaces; first and second clamp units whose front ends can open and close and form a ring-shaped body surrounding the clamped object; and an opening/closing mechanism for the clamp units. The first clamp unit is fixed to the main body at one side surface. The second clamp unit has a base end rotatably supported by a support shaft disposed toward the other side surface. The opening/closing mechanism includes: a biasing member biasing the second clamp unit to open the clamp units; and a sliding portion disposed on the other side surface and capable of sliding along the length of the main body. When slid toward the clamp units, the sliding portion presses the second clamp unit to close the front ends.