  We develop a formalism to carry out coarse-grainings in quantum field
theoretical systems by using a time-dependent projection operator in the
Heisenberg picture. A systematic perturbative expansion with respect to the
interaction part of the Hamiltonian is given, and a Langevin-type equation
without a time-convolution integral term is obtained. This method is applied to
a quantum field theoretical model, and coupled transport equations are derived.
