An object of the present invention is to provide a corrosion resistant steel excellent in strength and low temperature toughness as well as resistance to corrosion by carbon dioxide and seawater, and most suitable for oil well steel pipes and line pipes for production and transportation of gas, petroleum, etc. used in the field of energy, or a steel for plants, and corrosion resistant oil well steel pipes. The corrosion resistant steel and the corrosion resistant oil well steel pipes comprise, based on weight, up to 0.30% of C, up to 1.0% of Si, 0.2 to 2.0% of Mn, 2.1 to less than 5.0% of Cr, up to 0.03% of P, up to 0.02% of S, up to 0.10% of Al, up to 0.015% of N, optionally containing Cu, Ni, Mo, Ti, Nb and B, and the balance of Fe and unavoidable impurities, and have a martensitic structure as their metallic structure.