The present invention provides an efficiently usable honeycomb catalyst , a denitration catalyst of a denitration device, and an exhaust gas denitration device capable of reducing the running cost ofan exhaust gas denitration system by the denitration catalyst to approx. half. The honeycomb catalyst comprises gas flow passages for feeding treated gas from the inlet side to the outlet side thereof,performs treatment on the side walls of the gas flow passages, and has such a length that the flow of the treated gas fed in the gas flow passages is generally streamlined near the outlet side thereo f.