The "flux comparative method" was applied to the adsorption process, and an "adsorption characteristic function" was proposed. This method was originally proposed to predict the dependency on moisture content of the moisture transfer coefficient by using a drying rate curve with a constant material temperature. The relation between adsorption rate and mean adsorption amount was obtained from simulations for adsorption on a slab with negligibly small mass-transfer resistance in the surface film. The simulation was done with a diffusion coefficient that was exponentially dependent on adsorption amount. An analytical form (adsorption characteristic function) was proposed by which one can predict the relation between adsorption flux and the mean adsorption amount from the diffusion coefficient. The function was derived through the "flux comparative method", in which the steady-state flux for a mean adsorption amount is assumed to be proportional to the flux in a non-steady state with the same mean amount. Calculated results with the function showed a good agreement with the simulated ones.