  We develop a method of calculating the nonequilibrium steady state (NESS) of
an open quantum system that is weakly coupled to reservoirs in different
equilibrium states. We describe the system using a Redfield-type quantum master
equation (QME). We decompose the Redfield QME into a Lindblad-type QME and the
remaining part $\mathcal{R}$. Regarding the steady state of the Lindblad QME as
the unperturbed solution, we perform a perturbative calculation with respect to
$\mathcal{R}$ to obtain the NESS of the Redfield QME. The NESS thus determined
is exact up to the first order in the system-reservoir coupling strength
(pump/loss rate), which is the same as the order of validity of the QME. An
advantage of the proposed method in numerical computation is its applicability
to systems larger than those in methods of directly solving the original
Redfield QME. We apply the method to a noninteracting fermion system to obtain
an analytical expression of the NESS density matrix. We also numerically
demonstrate the method in a nonequilibrium quantum spin chain.
