We propose the magnetic exciton mediated superconductivity occurring in the enigmatic hidden-order phase of URu 2Si 2. The characteristic of the massive collective excitation observed only in the hidden-order phase is well reproduced by the antiferro hexadecapole ordering model as the trace of the dispersive crystalline-electric-field excitation. The disappearance of the superconductivity in the high-pressure antiferro magnetic phase can naturally be understood by the sudden suppression of the magnetic-exciton intensity. The analysis of the momentum dependence of the magnetic-exciton mode leads to the exotic chiral d-wave singlet pairing in the E g symmetry. The Ising-like magnetic-field response of the mode yields the strong anisotropy observed in the upper critical field even for the rather isotropic three-dimensional Fermi surfaces of this compound.