The present invention provides a peptide that consists of an amino acid sequence with 20 or fewer residues containing at least one amino acid sequence selected from the group consisting of the amino acid sequence represented in SEQ ID NO: 1, the amino acid sequence represented in SEQ ID NO: 2 and the amino acid sequence represented in SEQ ID NO: 38, the peptide having an activity to inhibit proliferation of a tumor cell or having an activity to inhibit binding between an epidermal growth factor receptor and a signal-transducing adaptor family member-2 (STAP-2); a peptide with a cell-penetrating peptide added to a terminus of the above peptide with or without a linker sequence interposed between the cell-penetrating peptide and the terminus; and pharmaceutical compositions containing any of these peptides.