  We present a derivation of the Markovian master equation by the
renormalization group method. Starting from a naive perturbative solution of
the von Neumann equation, the reduced density matrix with the coarse grained
time steps is obtained using the assumption of short correlation time of the
bath field. Then by applying the renormalization group method, we show that the
dependence of the specific initial time on the perturbative solution can be
removed and the Markovian semigroup master equation in the
Gorini--Kossakowski--Lindblad--Sudarshan (GKLS) form is obtained in the weak
coupling limit.
