PURPOSE: To form hardened layer having a small variance in hardness distribution, by adjusting suitably vol. ratio occupied by pearlitic phase in steel structure, then irradiating high energy heat source to a necessary part in surface of low alloy steel to quench the steel.

CONSTITUTION: Low alloy steel contg. about 0.4W0.9% C, about 0.10W2.0% Si, about 0.20W2.0% Mn, if necessary one or more kinds among ≤about 2% Cr, ≤about 5% Ni, ≤about 0.5% Mo is hot worked and stayed as it is or normalized to adjust vol. ratio occupied by pearlite phase in steel structure to ≥90% in surface of parts. Thereafter heat source of high energy density such as laser plasma is irradiated to the part requiring quenching processing of the parts. In this way, the irradiated part is transformed in a moment to uniform austenite layer, after the end of the irradiation, heat of source is radiated to inner part, the layer is cooled rapidly, and quenched to form hardened layer superior in quality.