An attempt to investigate the nuclear collective motion the standpoint of particle excitations is outlined. In our method it is possible to investigate the roles of the effective inter-particle interactions in the nuclear collective motions and clarify the mechanism of the collective excitation. The fundamental idea is illustrated by taking the simplified two dimensional harmonic oscillator shell model with the quadrupole-quadrupole effective inte ractions between particles. It is shown that this system successfully describes both the surface vibrational and the rotational collective motions consistently, depending upon the assumed configurations of particles in the shells. The striking success of the nuclear collective model in explaining the large variety of low-energy data has stimulated the interest of many workers in finding the method of quantum mechanical description of the nuclear collective motion. Various methods!) have been developed by many authors and have served us for understanding the various aspects of the nuclear collective motion. Most of these methods have been constructed on the supposition that some appropriate collective coordinates can be introduced in anticipation or that some specific collective modes of motion can be extracted from kinematical consider ations. In these methods, it seems that, the role of the effective inter-particle interactions as the origin of the collective motion has not been clarified satisfactorily. This problem will, however, be one of the most important problems to be investi gated urgently, at the present stage of the study of the nuclear unified model. It is our main purpose to outline an approach going beyond the limit of ap plicability of the above mentioned methods. According to our method developed in this paper, it is possible to investigate the role of the effective interactions in the nuclear collective motions and clarify the mechanism of the collective excitations. This possibility allows us to get a further insight into the various aspects of the unified model. In this paper we shall illustrate the fundamental ideas of our method by taking