A honeycomb catalyst body includes a tubular honeycomb base material having porous partition walls to define and form a plurality of cells extending as through channels of a fluid from one end surface from which the fluid flows in to the other end surface from which the fluid flows out, and a catalyst loaded onto the partition walls. In the honeycomb base material, at least one slit which is open in a side surface of the honeycomb base material is formed, and a width of the slit is from 1.0 to 10.0 mm.