PURPOSE: To harden exactly the range of the ball rolling surface to be hardened by bisecting a laser beam to two directions by a beam splitting mirror, reflecting the beams by reflection mirrors and irradiating simultaneously the beams to the right and left flanks of one screw thread on the surface of a screw shaft which is driven to rotate.

CONSTITUTION: The one laser beam 11 irradiated from a laser beam generator 10 is split to two laser beams 11a, b by a beam splitting mirror 12. The split beams 11a, b are reflected by respective reflection mirrors 13a, b and are simultaneously irradiated to the light and left flanks α, β of one screw thread D on the surface of a screw shaft A which is kept dipped at the lower half into cooling water 15 and is driven to rotate in this state, by which the surface is hardened. The shaft A is at the same time moved relatively with the laser beam generator 10.