We study liquid-gas transitions of heat conduction systems in contact with two heat baths under constant pressure in the linear response regime. On the basis of local equilibrium thermodynamics, we propose an equality with a global temperature, which determines the volume near the equilibrium liquid-gas transition. We find that the formation of the liquid-gas interface is accompanied by a discontinuous change in the volume when increasing the mean temperature of the baths. A supercooled gas near the interface is observed as a stable steady state.PACS numbers: 05.70. Ln,Introduction.-Liquid-gas transitions under constant pressure have been a classical subject of equilibrium thermodynamics [1]. In reality, however, a temperature gradient is formed, and thus the transition properties may be influenced by heat flow. As related experiments, enhanced heat conduction by condensation and evaporation was observed in turbulent systems [2,3]. In order to describe such nonequilibrium phenomena systematically, we first need to establish a thermodynamic theory for phase transitions under heat conduction.As the simplest situation, we consider cases where the pressure and heat flux are spatially homogeneous, which is illustrated in Fig. 1 [7][8][9][10]. Furthermore, since the density profile has to be determined under the constraint of global mass conservation, the variational principle for selecting the steady state, if it exists, should be formulated for the whole system. Such a theory has not been reported yet.Over the last two decades, statistical mechanics of nonequilibrium systems has progressed significantly [11][12][13], owing to the discovery of universal relations associated with the second law of thermodynamics [14][15][16][17][18][19][20]. As examples that may be related to the above problem, we point out extensions of thermodynamic relations [21][22][23][24][25][26], variational formulas associated with large deviation theory [27][28][29][30][31], representations of steady state probability densities [32][33][34], and inequalities stronger than the second law [35][36][37][38]. However, these results are not directly applicable to the analysis of liquid-gas transitions in heat conduction.In this Letter, we generalize an equilibrium variational principle that determines the volume near the liquid-gas transition. Concretely, on the basis of local equilibrium thermodynamics in the linear response regime, we propose the equality (11) with a global temperatureT , the main claim of this Letter, which corresponds to the generalized variational principle. This allows us to obtain the phase diagram of the heat conduction system, which can be examined in experiments.Setup.-We study the system shown in Fig. 1. A heat bath of temperature T 1 is attached to the left end (x = 0) of the system, and a second heat bath of temperature T 2 to the right end (x = L), where T 1 ≤ T 2 is assumed without loss of generality. We focus on cases that T c (p ex ) is far below the liquid-gas critical temperature. The length L of the system is fi...