A model of the interior of the heavy nucleus is proposed. Some experiments suggest that the proton pair and the neutron pair are moving in a strongly correlated manner inside of the heavy nucleus (alpha-correlation). To describe this, the proton-neutron interaction is extended from the pairing force to the many-body force. Accordingly, the BCS model must be extended to go beyond the Gorkov decoupling in order to incorporate the many-body force. As a result, comparing the pairing state, a more stable state where both pairing and the alpha correlation are developed is obtained. This model clarifies an interrelation between the paring and the alpha-correlation, because both are derived from a single interaction. The enhancement of the energy gap and the alpha correlation is derived from a common basis.