The pion is important as a mediator of nuclear force to bind nucleons in nucleus.  Because of the lack of theoretical framework to treat pion due to the pseudo-scalar nature of pion, we could not handle the pion explicitly for the calculation of nuclear structure.  We have developed a method to treat the pion in terms of the relativistic chiral mean field (RCMF) model, in which the ground state wave function is written in terms of the mean field part and 2p-2h excitations.  We demonstrate the RCMF to be able to treat the pion (tensor interaction) and show the major role of the pion to bind nucleons in nucleus.