Chemical potential is an index of a mass equilibrium condition in the equilibrium thermodynamics system. In this paper, a new function (water potential) based on chemical potential is proposed as the driving force of moisture flux. Thermodynamics functions are introduced and the chemical potential of a component of mixture gases is presented. They are applied to moisture phenomena, and then water potential is defined. The water potential is established by the Onsager's reciprocal theory to be the driving force of moisture flux, even in case of the nonequilibrium thermodynamics system (the non isothermal field). Furthermore, the propriety of the water potential is discussed by heat and moisture transfer experiments. The model room which is airtight and composed of two rooms, divided by a moisture permeable partition wall, is used for the experiments. As the result, if temperature is different between two rooms, absolute humidity is also different among those rooms and the partition wall in moisture equilibrium state, while the water potential is the same in them. It is suggested that the water potential is proper as the driving force of moisture flux.
