To provide a honeycomb catalyst body which is efficient in cleaning exhaust gas, keeps a high cleaning efficiency even when deteriorated during its using, is small in a pressure loss, and can be mounted in a limited space.

A honeycomb structure possesses porous partition walls forming a plurality of split cells which are flow passages of a fluid, and are arranged alternately to have the predetermined cells with the one end opened and the other end closed and the remaining cells with the one end closed and the other end opened. The honeycomb catalyst body keeps a catalyst carried by the inner surface of the pores of the partition walls of the honeycomb structure and by the surface of the partition walls, and the density of the cells of the section orthogonal to the center axis of the honeycomb structure is 300 cpsi or more and less than 750 cpsi.