We study the characteristics of the tensor correlation in 4 He using a shell model type method. We treat the tensor force explicitly by performing a configuration-mixing calculation in the 2p2h basis and include single-particle states up to intermediately high angular momenta. We adopt the Gaussian expansion method for the quantitative description of the spatial shrinkage of the single-particle states to optimize the tensor correlation. We are able to describe the full strength of the tensor correlation for 4 He in the shell model type method by realizing convergence. We call this model the tensor-optimized shell model. It is found that in 4 He, three specific 2p2h configurations are strongly coupled with the (0s) 4 configuration due to the characteristic features of the tensor operator. * ) systems use the relative coordinates of nucleons, which are suitable to work out the nucleon-nucleon interaction. We call this the many-body theory with T -type basis, because the two interacting nucleons, whose centers of mass are connected by some reference coordinate, interact directly through the nucleon-nucleon interaction. In this case, the number of relative coordinates increases as A(A − 1)/2, where A is the number of nucleons. Hence, the T -type basis is not suited for heavier systems, and calculations become increasingly difficult as A increases. By contrast, the meanfield framework uses as the coordinates the positions of the nucleons relative to the center of the nucleus. In this case, the number of coordinates is A, and hence this framework is suited for heavy systems. We call this method the many-body theory with V -type basis, since the two interacting nucleons are labeled by the coordinates representing the distances from the center of the nucleus, and they interact only indirectly through the nucleon-nucleon interaction. Hence, it is difficult to treat realistic interactions involving short-range repulsion and the tensor force, because many configurations in the V -type basis are needed to describe motion under such a realistic interaction. In this case, we have to invent some method to treat these features of realistic interactions.The standard method for the description of many-body systems in the V -type basis is the Brueckner-Hartree-Fock (BHF) method, 5)-7) in which two-body interactions with a short-range repulsion and a strong tensor force are treated with the Brueckner G-matrix effective interaction under the independent pair approximation. Since the resulting G-matrix effective interaction is a smooth interaction, the wave functions in the V -type basis can treat the G-matrix within the model space. The BHF method or the shell model, which is based on the Brueckner theory, has some success in the description of many-body systems. However, there seem to be some essential features missing, such as the spin-orbit interaction 8), 9) and we are therefore forced to employ some phenomenology.Recently, there have been two important steps proposed for the full description of nuclei in the V -type basis. One i...