The invention provides a heat resisting steel having superior high-temperature strength and notch rupture strength, a rotor shaft using the heat resisting steel, a steam turbine using the rotor shaft, and a power plant using the steam turbine. The heat resisting steel is made of a Cr--Mo--V low-alloy steel containing 0.15-0.40% by weight of C, not more than 0.5% of Si, 0.05-0.50% of Mn, 0.5-1.5% of Ni, 0.8-1.5% of Cr, 0.8-1.8% of Mo and 0.05-0.35% of V, and having a (Ni/Mn) ratio of 3.0-10.0.