A method formally analogous to that developed by Bogoliubov, Tolmachev and Shirkov to investigate the collective excitations in superconductors is applied to even-even spherical nuclei in order to investigate the mechanism of the nuclear collective motion from the standpoint of particle excitations. Our theory leads, in principle, to the same results as those obtained by Belyaev. However, the method of description of the nuclear collective motion is quite different from that of Belyaev's paper in which the “cranking model” of Inglis is employed, and the various physical parameters used by Belyaev can be derived uniquely from the “first” principle. Thus, in so far as the vibrational motion is concerned, the physical implication underlying the nuclear collective model proposed by Bohr and Mottelson is made clear. It is outside the scope of this paper to relate the effective inter-particle interactions used in this paper with the nuclear forces known from the two nucleon problems.