  We develop the effective field theory of diffusive Nambu-Goldstone (NG) modes
associated with spontaneous internal symmetry breaking taking place in
nonequilibrium open systems. The effective Lagrangian describing semi-classical
dynamics of the NG modes is derived and matching conditions for low-energy
coefficients are also investigated. Due to new terms peculiar to open systems,
the associated NG modes show diffusive gapless behaviors in contrast to the
propagating NG mode in closed systems. We demonstrate two typical situations
relevant to the condensed matter physics and high-energy physics, where
diffusive type-A or type-B NG modes appear.
