A mucosal absorption-enhancing agent is provided that enables oral, nasal or pulmonary administration of peptide drugs whose administration route has heretofore been limited to the injections due to their poor absorption from the mucosa. Specifically, the mucosal absorption of peptide drugs via intestinal, pulmonary or nasal route can be enhanced by allowing the peptide drugs with the C-terminal fragment (C-CPE) of an enterotoxin (CPE) produced by the bacterium Clostridium perfringens of the genus Clostridium, in particular with the C-CPE or its mutants resulting from the substitution and/or deletion of one or several amino acid residues of the C-CPE to act thereon. The composition for mucosal administration of the present invention significantly enhances absorption of peptide drugs, such as human parathyroid hormone hPTH(1-34), human ghrelin and human motilin, through the mucosa of small intestine, lung, nasal cavity and other mucosa. Also, unlike any of the conventional mucosal absorption-enhancers, the composition for mucosal administration of the present invention does not cause tissue damage and is therefore highly safe for use.