  By considering a thermodynamic force as gauge field, we extend constitutive
equations of Onsager's non-equilibrium thermodynamics to non-linear equations.
In Onsager's non-equilibrium thermodynamics, the thermodynamic force
corresponds to a pure gauge, for which the constitutive equations are obtained
by gauge fixing. If we extend the thermodynamic force from pure gauge to
physical one, we obtain the non-linear constitutive equations of
non-equilibrium thermodynamics.
