  We consider decay processes of scalar-field condensation in the framework of
well-established quantum field theory. We postulate that the quantum state
corresponding to the scalar-field condensation is so-called coherent state with
discussing the validity of such a treatment. We show that, by using the
unitarity relation of the scattering matrix, decay rate of the coherent state
is systematically calculated. We apply our procedure to derive explicit
formulae of decay rates for two cases: (i) we study the case where the scalar
condensation decays into a pair of scalar particles and show that our formalism
reproduces the results obtained from the parametric-resonance analysis, and
(ii) we also calculate the decay rate when the coherent state decays via
anomaly.
