i53 In a certain type of Calabi-Yau superstring models it is clarified that the symmetry breaking occurs by stages at two large intermediate energy scales and that two large intermediate scales induce large Majorana-masses of right-handed neutrinos. Peculiar structure of the effective nonrenormalizable interactions is crucial in the models. In this scheme Majorana-masses possibly amount to 0(10 9 -10 GeV) and see-saw mechanism is at work for neutrinos. Based on this scheme we propose a viable

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... del which explains the smallness of masses for three kinds of neutrino lie, lip and llr. Special forms of the nonrenormalizable interactions can be understood as a consequence of an appropriate discrete symmetry of the compactified manifold.