[en] This is the third in a series of papers which intends to develop a new microscopic theory capable by itself to determine the ''maximal-decoupled'' collective subspace in the many-particle Hilbert space as well as the collective Hamiltonian specifying the subspace. The fundamental principle to formulate the theory is called the Invariance principle of the Schroedinger equation. The main purpose of this paper is to show that, when we positively utilize the invariance principle, we can directly obtain a quantum theory of collective motion going beyond the random phase approximation. (author)