Enhancement of adsorption/desorption rate in an adsorption heat exchanger is effective to enhance cooling/heating power per unit volume of adsorption chillers and dehumidifying/humidifying rate of desiccant systems. To achieve this, it is necessary to improve heat transfer performance in the adsorption heat exchanger. In the present study, for a porous alumina film adsorbent synthesized with anodization on an aluminum plate, we measured the adsorption/desorption rate in water vapor with gravimetric method using a cantilever. Consequently, the effective diffusivity in water vapor was larger than that in moist air due to the mass transfer resistance of fluid film in moist air. In water vapor, the diffusivity inside pores dominated the diffusivity and it decreased when the number of adsorption layer changes. It is supposed that a change of activation energy for adsorption/desorption due to a change of the adsorption layer number leads to the decrease of the diffusivity.