Provided is a novel cancer therapeutic agent which can be used as a novel choice for the treatment of cancer. Specifically provided are: a peptide which comprises a binding site for a PHB2 polypeptide in an ERAP1 polypeptide and can inhibit the binding between the ERAP1 polypeptide and the PHB2 polypeptide a pharmaceutical composition containing the peptide and a method for screening for a drug candidate for treating and/or preventing cancer, in which the inhibition of the binding between an ERAP1 polypeptide and a PP1α polypeptide, a PKA polypeptide or a PKB polypeptide is employed as a measure.