An excitation core includes a first strip-shaped magnetic substance and a second strip-shaped magnetic substance. The first strip-shaped magnetic substance has both ends mutually overlapped with and mutually in contact with both ends of the second strip-shaped magnetic substance in a thickness direction. A sensor head includes this excitation core and a magnetism collecting body. This magnetism collecting body includes strip-shaped first magnetism collector and second magnetism collector. Both ends of the first magnetism collector and both ends of the second magnetism collector are in contact with one another to form the annular magnetism collecting body. This magnetism collecting body is disposed at an inside or an outside of the reinforcing body.