There is provided an inductor for heating the inner circumference of a hole, by which induction heating of the inner circumference of a hole and so on can be realized rapidly and uniformly in the direction of the circumference, and there is no risk of a short-circuit between a solenoidal coil and transition conductor passing therethrough. Each plate has an active area, a header area, and a perimeter area. The body has a central axis, a top portion and a bottom portion. 0 mol %, zinc oxide : more than 12.