  We propose a mechanism to describe how a physical quantity, which initially
can take continuous values, is restricted within some discrete values after a
measurement. As an example of the present theory, in which interplay between
coherence of motion and fluctuation from disturbance plays an important role,
we investigate motion of a spin state in a magnetic field. First, we point out
that discrete eigenstates are formed from continuous states as a result of
coherence of precession motion of a spin. Next, by assuming disturbance from
environmental electromagnetic fields, we investigate temporal change of
direction of a spin state by applying the first order perturbation theory and
the Monte Carlo technique. Results of simulations show that the spins, whose
directions are randomly distributed at the initial time, are reoriented toward
only two directions due to fluctuation guided by coherence.
