  We give a proof of the recursive formula on the norm of Whittaker vector of
the deformed Virasoro algebra, which is an analog of the one for the Virasoro
Lie algebra proposed by Al. Zamolodchikov. Our formula gives a proof of the
pure SU(2) 5d AGT relation proposed by Awata and Yamada. We also give a summary
of the structures of the deformed Virasoro algebra and the fundamental
properties of the Verma module.
