The stainless steel 1Cr18Ni9Ti corrodes severely in the boiling 45% MgCl_2 solution. But an improvement on strength and prolongation of rupture time under stress corrosion are obviously shown if it undergoes an application of alternating stress, saying, 20 to 39 MPa, superposing to the static stress. The smaller the static stress, the more obvious the effect is. Even the alternating stress increases up to 68 MPa, it does not shorten any more rupture time. Since the machine parts are quite difficult to operate under no mechanical vibration, it seems to be available for elongating the rupture life of which the machinery runs under corrosive medium with an additional alternating stress controlled within a narrow limit.