  We investigate QCD in the canonical ensemble with respect to the quark
number. We reveal that the canonical description in which the quark number is
fixed would be reduced to the grand canonical description under the
thermodynamic limit. Since the grand canonical ensemble contains fluctuations
of the quark number, the idea of the canonical ensemble is of no use for the
purpose of defining order parameters for the deconfinement transition. We
clarify the origin of such reduction and propose an idea to define the order
parameter. The Monte-Carlo simulation by means of the spin system, which is an
effective model of QCD at finite temperature, shows prosperous behaviors though
the results suffer from the severe statistical error due to the sign problem.
