Poly(itaconic acid) esters with oligo(ethylene oxide) side chains can form amorphous solid solutions with LiClO4 and NaClO4. Many research groups have examined AC ionic conductivities of these polymer/salt mixtures as a function of side-chain length and salt concentration. We propose a model to interpret these experimental data based on a configurational entropy consisting of three kinds of entropy such as a mixing of polymer and salt ions, a disorientation of polymers, and a drop of chain flexibility (or increase of T-g) with salt concentrations. Quantitative description according to the proposed model is presented.