PURPOSE: To form a hardened surface layer of a uniform depth in the whole of a work by arranging plural high frequency heating coils having a prescribed shape so that the circular surface of the work can be heated with the separate coils.

CONSTITUTION: Thin semiopen induction hardening coil is composed of a first heating coil 51, a second heating coil 52 having the same shape as the coil 51, a coil 53 connecting one end of the coil 51 to one end of the coil 52, a pair of power supplying coils 54, 55 and a core 59 and high frequency voltage 40 is impressed on ends of the coils 54, 55. The first coil 51 is positioned at one side of the circular surface of a work 10 to be hardened and has the shape of a circular are corresponding to about 1/4 of a circle. The core 59 is formed so as to cover the side faces of the coils 51, 52, 53 not confronting the circular surface to be hardened.