A new method of calculating concentration dependent mutual diffusion coefficients was proposed for the penetration period of adsorption and desorption processes in polymer film. The diffusion equation for a slab based on a dissolved solid coordinate with constant surface concentration was solved numerically for the sorption process, assuming a power function for the concentration dependence of the diffusion coefficient. The solvent concentrations corresponding to the apparent diffusion coefficients, which were calculated easily from the experimental sorption rate curves of the penetration period, were correlated with the surface concentrations using the power value of the concentration dependence as the parameter. Using this correlation, the relation between the diffusion coefficient and the concentration was obtained easily by trial calculation.