The effect of hydrostatic pressure on the solubility of tetragonal hen egg-white lysozyme crystals was measured in situ by a two-beam interferometer combined with a high-pressure optical cell which was newly designed by our group. The change in the concentration with time during equilibration was measured accurately and continuously starting from a supersaturated state (growth relaxation) and an undersaturated state (dissolution relaxation). The solubility value thus measured at 100 MPa was 28% larger than that at 0.1 MPa, and this increase was much smaller than those of previous reports.