A sensor which is adapted to attach to a head of a subject includes a first fastener that includes a first end portion and a second end portion and extends into an arc-shape, a first coupling portion that is disposed on an inner circumference side of the first fastener, a second fastener that has a first through hole and a second through hole, a second coupling portion that is disposed on a first side of the second fastener, and attachable to and detachable from the first coupling portion, a light emitter that is disposed on the first side of the second fastener, and opposed to the first through hole, and a light detector that is disposed on the first side of the second fastener, and opposed to the second through hole.
