The present invention relates to an agent for inhibiting decomposition of a biologically active peptide by a digestive enzyme, which is aminoalkyl methacrylate copolymer E as an effective ingredient, and an agent for reducing viscosity of the gastrointestinal mucosa and mucous layers distributed over this mucosa. The present invention relates to a method for inhibiting decomposition of a biologically active peptide by a digestive enzyme, or a method for reducing viscosity of the gastrointestinal mucosa and/or mucous layers distributed over the mucosa, by administration of aminoalkyl methacrylate copolymer E. Furthermore, the present invention relates to a pharmaceutical composition for oral use with improved absorption, which comprises a biologically active peptide and aminoalkyl methacrylate copolymer E. Excellent oral absorption of a biologically active peptide that were considered difficult to absorb by oral administration in the past, can be realized based on the ability to inhibit decomposition of a biologically active peptide, the ability to reduce viscosity of the gastrointestinal mucosa and/or mucous layers distributed over this mucosa, and the ability to improve permeability of a biologically active peptide in the gastrointestinal mucosa and/or mucous layers of the pharmaceutical composition of the present invention.