  A new proof of the direct part of the quantum channel coding theorem is shown
based on a standpoint of quantum hypothesis testing. A packing procedure of
mutually noncommutative operators is carried out to derive an upper bound on
the error probability, which is similar to Feinstein's lemma in classical
channel coding. The upper bound is used to show the proof of the direct part
along with a variant of Hiai-Petz's theorem in quantum hypothesis testing.
