[en] The importance of the monopole component of effective NN interaction has been pointed out since 1960's in the context of energy difference between different shell-model configurations [1-3]. It has been more emphasized in recent studies on shell structure evolution in neutron-rich nuclei [4-6]. The definition of single-particle (s.p.) energy is not trivial, especially for open-shell nuclei. According to the prescription of Baranger [7], we derived an expression of s.p. energies by employing a shell-model sum rule technique [5]. The result is very simple and reasonable. The s.p. energy can be evaluated for any state by a sum of ε_j^core with respect to the core nucleus and a part which arises from interaction between valence nucleons by calculating occupation numbers of s.p. orbits. The s.p. energies are determined only by the monopole strengths of NN interaction. This formulation allows us, for the first time, to discuss quantitatively changes of s.p. energies from a nucleus to another due to the monopole interaction, and, consequently, the evolution of shell structure. We have studied the monopole interaction in detail [6]. It is the lowest order term in the multipole expansion of NN interactions. The spin-tensor decomposition revealed that the monopole strengths are dominated by the triplet-even attraction of the central force, which contains second-order effects of the tensor force. The two-body monopole interaction is renormalized, as a very good approximation, to the one-body s.p. energies, which indicates that the above formulation of s.p. energies is consistent with the two-body interaction. The monopole, therefore, dominates the nuclear binding due to NN interactions. However, since the monopole interaction is expressed by number operators, it is diagonal with respect to shell-model configurations, and further binding is gained mainly by the quadrupole component. A study is under progress on the roles of central, tensor and spin-orbit components of NN interactions in a wide range of nuclear chart, with an emphasis on the monopole interaction.(author)