  We have investigated the nature of the antiferromagnetic (AF) phase induced
by uniaxial stress sigma in URu2Si2, by performing elastic neutron scattering
measurements up to 0.4 GPa. We have found that the AF Bragg-peak intensity
shows a clear hysteresis loop with sigma under the zero-stress cooling
condition. The result strongly suggests that the sigma-induced AF phase is
metastable and separated from the coexisting "hidden ordered" phase by a
first-order phase transition. We also present the analyses of the crystalline
strain effects, and suggest that the c/a ratio plays an important role in the
competition between these two phases.
