A contribution to internucleon potentials due to many-body forces in heavy nuclei is calculated from semi-phenomenological πN interactions using non-relativistic perturbation theory. We consider p-wave interactions only in the lowest order, while s-wave interactions are treated by taking their average over a Fermi distribution at the outset. This approximation leads to a large increment of the effective mass of pion. The Pauli principle is taken into account for the intermediate states. The resultant effective two-body potentials is shown graphically.