The present invention presents a pharmaceutical composition for oral use with improved absorption, which comprises drug, aminoalkyl methacrylate copolymer E, and acidic substance and is obtained by bringing said 3 components together and uniformly mixing at least this polymer and this acidic substance, and a method of improving oral absorption by using this pharmaceutical composition. Moreover, the present invention presents an agent for improving oral absorption that increases drug permeability of the digestive tract mucous membrane and/or mucous layer present on the surface of this membrane, whose active ingredient is aminoalkyl methacrylate copolymer E. In addition, the present invention presents an oral agent for improving absorption by increasing drug permeability of the digestive tract mucous membrane and/or the mucous layer distributed over this mucous membrane, whose effective component is aminoalkyl methacrylate copolymer E.