PURPOSE:To form a uniformly hardened layer on the surface of a metal having a wide area by relatively moving a local heating part by a plasma arc considerably in the transverse direction of hardening at the time of hardening the surface of a metallic material by the plasma arc. CONSTITUTION:A plasma torch 1 is moved back and forth in an arrow 7 direction and at the same time the round-bar-shaped metallic material 3 is intermittently turned in an arrow 8 direction at the time of hardening the surface of the material 3 by the plasma arc 2 generated by the plasma torch 1 to form a heating locus 9 in a range of a hardening width 5. The local heating part 2' of the plasma arc 2 is considerably moved in the transverse direction of hardening and the metallic stock 3 is heated to the hardening temp. or above respectively locally at the overall length of the hardening width 5 thereof. The locus number 9... of the movement of the plasma arc relative to the stock 3 are successively increased in the longitudinal direction 8' of hardening while said numbers are successively paralleled with the previous loci. The relative moving speed of the stock 3 and the plasma arc 2 is set to the hardening temp. or above. The space between the adjacent moving loci is set at such a width at which the hardening temp. zones are continuous.
