Abstract: A martensitic stainless steel for use in oil wells has improved stability in strength and good resistance to sulfide stress corrosion cracking The steel has a chemical composition consisting essentially, on a weight basis, of: Si: not greater than 1.0%, Mn: not greater than 1.0%, Cr: 10.0-14.0%, Mo: 0.5-7.0%, Ni: 4.0-8.0%, Al: 0.001-0.1%, either Ti in an amount satisfying the following inequality (1) or Zr in an amount satisfying the following inequality (2), optionally one or both of Mg: 0.001-0.05% and Ce: 0.001-0.05%, and a balance of Fe and incidental impurities in which the contents of C, P, S, N, and V as impurities are limited to 0.05% or less, 0.04% or less, 0.005% or less, 0.05% or less, and 0.2% or less, respectively, the composition further satisfying the following inequalities (3) and (4):4(%C).ltoreq.%Ti.ltoreq.{-0.01/(%C+0.015))}+0.75 (1)10(%C).ltoreq.%Zr.ltoreq.2 0% (2)30(%Cr)+36(%Mo)+14(%Si)-28(%Ni).ltoreq.455 (3)21(%Cr)+25(%Mo)+17(%Si)+35(%Ni).ltoreq.731 (4).