A probe adapted to be used with a pulse oximeter is disclosed. A flexible first housing is adapted to be brought into contact with at least a nail of a finger or a toe of a subject. A flexible second housing is adapted to be brought into contact with at least a top of the finger or the toe. A flexible connecting part connects the first housing and the second housing, and is adapted to cover a tip end of the nail. A light emitting element is provided on one of the first housing and the second housing. A light receiving element is provided on the other one of the first housing and the second housing.