Provided is a novel cancer-treating agent which can be used as a novel choice for the treatment of cancer. Specifically provided are: a peptide that inhibits binding of ERAP1 polypeptide to PHB2 polypeptide, which comprises a binding site of the ERAP1 polypeptide to the PHB2 polypeptide, and a pharmaceutical composition comprising the peptide. In addition, provided is a method for screening a drug candidate for treating and/or preventing cancer using inhibition of the binding of the ERAP1 polypeptide to PP1α polypeptide, PKA polypeptide or PKB polypeptide as an index.