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The thermo-hydraulics in a thermal non-equilibrium state, which may appear during a postulated loss-of-coolant accident, is one of phenomena which have never been understood enough. In this paper, accelerating boiling two-phase flow is dealt with which realizes thermal non- equilibrium conditions in a steady state in order to clarify the relaxation process of superheated liquid. The following insight is clarified from the results; Jones' model cannot explain the underpressure and relaxation time unless an empirical constant is changed. Therefore, the effect of interfacial area density on the macroscopic relaxation inception is experimentally investigated by adding artificially bubbles in accelerating boiling two-phase flow in order to offer a fundamental data base for modeling the relaxation process of superheated liquid. It is made clear that the macroscopic relaxation inception can be evaluated by a relationship between the integration of liquid superheating by the travelling time and the initial interfacial area density.