The possibility for occurrence of a Bose condensate of dibaryons in nuclear matter is investigated within the framework of the Walecka model in the mean-field approximation. Constraints for the omega - and sigma - meson coupling constants with dibaryons following from the requirement of stability of dibaryon matter against compression are derived and the effect of sigma - and pi - meson exchange current contributions to the sigma - dibaryon coupling constant is discussed. The mean-field solutions of the model are constructed. The effective nucleon mass vanishes when the density of dibaryons approaches a critical value about 0.15/fm^3. The Green's functions of the equilibrium binary mixture of nucleons and dibaryons are constructed by solving the Gorkov-Dyson system of equations in the no-loop approximation. We find that when the square of the sound velocity is positive, the dispersion laws for all elementary excitations of the system are real functions. This indicates stability of the ground state of the heterophase nucleon-dibaryon mixture. In the model considered, production of dibaryons becomes energetically favorable at higher densities as compared to estimates based on a model of non-interacting nucleons and dibaryons.

35 pages, 15 figures