The nucleus is an isolated finite many-body quantum system, in which the se
lf-consistent mean-field is realized. The system reveals the characteristic
facets of the surprising coexistence of "macroscopic" and "microscopic" ef
fects in association with various "phase transitions". Contrary to the infi
nite many-body systems at the macroscopic level in which the phase transiti
on happens sharply, the nucleus as the finite quantum system requires for u
s to develop a microscopic theory of analyzing the phase transition in term
s of the individual quantal states on the basis of the nonlinear dynamics.
The characteristic difficulties in developing the microscopic theory rest o
n the following fact: Because of the finite self-bound system, in the nucle
us, the single-particle modes of motion (more generally, non-collective deg
rees of freedom) have to adjust their features self-consistently, in accord
ance with the time-evolution of the mean-field associated with the large-am
plitude collective motion. Keeping the strong self-consistency between the
single-particle motion and the collective motion, one has to develop the mi
croscopic theory which is capable of explaining appearance, persistency, tr
ansfiguration and termination of the large-amplitude collective motion. We
hope that such a challenge may provide us to explore "quantum mechanical th
eory of nonlinear dynamics", and give a clue to explain the thermal feature
in isolated systems, as a result of the reversible process without any sta
tistical hypothesis. In this paper, the efforts so far made by us are revie
wed, towards the microscopic theory of the nonlinear dynamics of nuclear co
llective motion.