Cyclic peptide or salt thereof and MDMX inhibitor

The present invention addresses the problem of providing a cyclic peptide or salt thereof having excellent cell membrane permeability and excellent target bindability with MDMX, and an MDMX inhibitor. The present invention provides a cyclic peptide, or a salt thereof, which is represented by formula (1) defined in the specification of the present application, and which has features (a)-(d) below. (a) The cyclic peptide has a structure in which, when a is taken to be the axial length in the axial direction of the longest main chain structure and b and c are taken to be the axial lengths in two other directions which are orthogonal to a and orthogonal to each other, and the ellipsoid approximation is performed to determine the axial lengths for a, b, and c, and through this approximation stage, the molecular shape factor r is calculated using formula (2), r ranges from 0.4 to 0.6. (b) The aforementioned peptide is nonionic under a physiological environment. (c) The proportion of the number of N-substituted amin