A curving section is always disposed in a projection plane of a probe main body section. The curving section includes a first bending surface which bends relative to a peripheral surface of the probe main body section to approach a longitudinal axis, thereby intersecting the longitudinal axis a second bending surface which bends relative to the first bending surface toward a bending direction of the first bending surface and a direction away from the longitudinal axis a third bending surface which bends relative to the second bending surface toward a direction to approach the longitudinal axis on a side reverse to the bending direction of the first bending surface, and extends toward an extension line of the first bending surface.
