The present invention provides an age hardening type bainitic microalloyed steel having a composition which includes, in terms of mass %: 0.06-0.35% of C; 0.01-2.00% of Si; 0.10-3.00% of Mn; 0.001-0.200% of S; 0.001-2.00% of Cu; 0.40-3.00% of Ni; 0.10-3.00% of Cr; 0.10-1.00% of Mo; 0.10-1.00% of V; and 0.001-0.100% of s-Al, with the remainder being Fe and unavoidable impurities, and which satisfies a value of the following expression (1) to be 20 or larger and a value of the following expression (2) to be 0.82 or larger: 3×[C]+10×[Mn]+2×[Cu]+2×[Ni]+12×[Cr]+9×[Mo]+2×[V]  expression (1); 1.66×[C]+0.18×[Si]+0.27×[Mn]+0.09×[Ni]+0.32×[Cr]+0.34×[Mo]+0.44×[V]  expression (2), in which each [ ] in the expression (1) and the expression (2) indicates a content of the element shown therein in terms of mass %.