  We present numerical calculations of the partial width of the charged Higgs
boson decay into a top quark, $H^- \rightarrow \bar{t} + b + X$, and the
partial width of the top quark decay into a light charged Higgs boson $t
\rightarrow H^+ + b + X$ at next-to-next-to-leading order (NNLO) in QCD, based
on a factorization formula of the jet mass. The NNLO corrections significantly
reduce the renormalization scale dependence of the partial decay width in both
cases. We show relative size of the NNLO corrections for different charged
Higgs boson masses and for different renormalization scales. The NNLO
corrections are about 16% (1%) of the leading order widths for the charged
Higgs boson mass of 200GeV (2000GeV), while it is quite small for the top quark
decay. Our analyses are independent of the detailed structure of the Yukawa
couplings, and can be applied to various new physics models, as demonstrated by
the decay branching ratio in different types of the two-Higgs-doublet models.
