A honeycomb catalyst body capable of containing in a limited space, having high mechanical strength, less pressure loss, and suitable purification performance is provided, and it includes a plugged honeycomb structure having porous partition walls defining cells, plugged portions, and porous projecting portions projecting so as to extend from the partition walls into the cells and being formed integrally with the partition walls; and catalysts loaded onto the projecting portions, wherein a porosity of the partition walls is 40 to 70%, thicknesses of the projecting portions is from 30 to 140% of thicknesses of the partition wall, an amount of the catalyst loaded onto the partition walls thereof is smaller than an amount of the catalyst loaded onto the projecting portions, one catalyst loaded onto projecting portions is a selective reduction catalyst, and another catalyst loaded onto projecting portions is an ammonia slip preventing catalyst.