  We give a brief review on the important role of the pion-exchange interaction
characterized by the strong tensor force. In order to illustrate the role of
the pion, we have proposed a new mean field theory with the charge and parity
mixing, by which the pion is treated explicitly in the mean field framework. We
introduce the essence of the new mean field theory and illustrate the
calculated results of the pion (tensor force) correlation by taking the alpha
particle as an example.
