Stimuli-responsive gels are the polymer gels whose volume is varied according to environmental conditions. It has been reported that stimuli-responsive gels with an inhomogeneous structure exhibit faster volume changes than gels without such structures. It is understood as a difference in the transfer dynamics of the solvent, though, there are few models for discussing the effect of inhomogeneity explicitly. In this paper, we propose a simple model for the kinetics of volume change by introducing inhomogeneity as the probability distribution of a random variable that characterizes the structure. Under this framework, we demonstrate that inhomogeneity actually increases the rate of volume change.