PURPOSE: To form a chill layer of a uniform depth by generating a magnetic field from a magnetic coil during irradiation with an energy beam at the time of irradiating the prescribed surface area of a work with the energy beam thereby executing the hardening treatment of the work by remelting.

CONSTITUTION: A cam surface 22 of a cam shaft 20 and a beam torch 26 are relatively moved and the prescribed surface area of the cam surface 22 is irradiated with the energy beam 27 from the beam torch 26 and is remelted, by which the hardening treatment is executed at the time of subjecting the cam shaft 20 to the hardening treatment by remelting. The magnetic coil 30 is fitted nearly concentrically onto to the outside of the beam torch 26 at this time and the magnetic field 38 is generated by supplying an AC current from an AC power source 31 to the magnetic coil 30 at the time of irradiating the cam surface 22 with the energy beam 27 from the beam torch 26. The molten metal receives Lorentiz's force and solidifies under stirring. The chill layer 34 of a uniform depth is thus formed over the entire part.