Abstract: A heat resisting steel suitable for use as the materials of steam turbine casings of steam turbines for thermal power generation. The steel has a composition consisting essentially of 0.05 to 0.2 wt % of C, 0.5 to 2.0 wt % of Cr, 0.5 to 2.0 wt % of Mo, 0.05 to 0.5 wt % of V, 0.002 to 0.1 wt % of Al, 0.0002 to 0.0030 wt % of B and the balance substantially Fe and inevitable impurities, wherein the sum of X and Al as obtained from the following formulae is not greater than 2920:X=10P+5Sb+4Sn+AsAl=xAl(x being a coefficient obtained from FIG. 4)where, P,Sb,Sn and As represent the contents of P,Sb,Sn and As contained as said inevitable impurities in terms of ppm, while Al represents the Al content in terms of ppm. This heat resisting steel exhibits a large resistance to cracking in stress relief annealing after welding, as well as a high creep rupture strength at high temperature, and shows only a low crack developing speed.