PURPOSE: To easily obtain a tough and hard steel for machine structural use excellent in cold forgeability by specifying a composition consisting of C, Si, Mn, Cr, Cu, P, S, O, and Fe.

CONSTITUTION: This tough and hard steel for machine structural use has a composition consisting of, by weight, >0.45W0.60% C, <0.02% Si, 0.40W0.70% Mn, 0.35W1.20% Cr, ≤0.02% Cu, ≤0.01% P, ≤0.01% S, ≤0.002% O, and the balance essentially Fe and satisfying 5.0≤(1+4.1Mn)(1+2.33Cr)≤10, and this steel is excellent in all of various cold forging characteristics, such as low deformation resistance, high hardenability, and surface oxidation resistance.