A honeycomb catalyst body (100, 101) comprising: a plugged honeycomb structure having porous partition walls which define and form a plurality of cells extending from an inflow side end surface to an outflow side end surface to become through channels of a fluid, inflow side plugged portions provided at inflow side ends of the predetermined cells, outflow side plugged portions provided at outflow side ends of the remaining cells, and porous projecting portions (9, 19) which project so as to extend from the partition walls into inflow cells, that are defined as the cells provided with the outflow side plugged portions, and into outflow cells, that are defined as the cells provided with the inflow side plugged portions, and which are formed integrally with the partition walls; and a catalyst loaded onto the projecting portions (9, 19) of the plugged honeycomb structure, wherein a porosity of the partition walls of the plugged honeycomb structure is from 40 to 70%, a thickness of the projecting portions (9, 19) is from 30 to 140% of a thickness of the partition walls, an amount of a catalyst loaded onto the partition walls in a cross section vertical to an extending direction of the cells is 5% or less of a total amount of the catalyst loaded onto the plugged honeycomb structure, and the amount of the catalyst loaded onto the partition walls is smaller than an amount of the catalyst loaded onto the projecting portions (9, 19), and the catalyst loaded onto the projecting portions (9a, 19a) extending into the inflow cells of the plugged honeycomb structure is a three-way catalyst or an NO x adsorber catalyst, and the catalyst loaded onto the projecting portions (9b, 19b) extending into the outflow cells of the plugged honeycomb structure is a selective reduction catalyst, or the catalyst loaded onto the projecting portions (9a, 19a) extending into the inflow cells of the plugged honeycomb structure is a three-way catalyst, and the catalyst loaded onto the projecting portions (9b, 19b) extending into the outflow cells of the plugged honeycomb structure is an NO x adsorber catalyst. a plugged honeycomb structure having porous partition walls which define and form a plurality of cells extending from an inflow side end surface to an outflow side end surface to become through channels of a fluid, inflow side plugged portions provided at inflow side ends of the predetermined cells, outflow side plugged portions provided at outflow side ends of the remaining cells, and porous projecting portions (9, 19) which project so as to extend from the partition walls into inflow cells, that are defined as the cells provided with the outflow side plugged portions, and into outflow cells, that are defined as the cells provided with the inflow side plugged portions, and which are formed integrally with the partition walls; and a catalyst loaded onto the projecting portions (9, 19) of the plugged honeycomb structure, wherein a porosity of the partition walls of the plugged honeycomb structure is from 40 to 70%, a thickness of the projecting portions (9, 19) is from 30 to 140% of a thickness of the partition walls, an amount of a catalyst loaded onto the partition walls in a cross section vertical to an extending direction of the cells is 5% or less of a total amount of the catalyst loaded onto the plugged honeycomb structure, and the amount of the catalyst loaded onto the partition walls is smaller than an amount of the catalyst loaded onto the projecting portions (9, 19), and the catalyst loaded onto the projecting portions (9a, 19a) extending into the inflow cells of the plugged honeycomb structure is a three-way catalyst or an NO x adsorber catalyst, and the catalyst loaded onto the projecting portions (9b, 19b) extending into the outflow cells of the plugged honeycomb structure is a selective reduction catalyst, or the catalyst loaded onto the projecting portions (9a, 19a) extending into the inflow cells of the plugged honeycomb structure is a three-way catalyst, and the catalyst loaded onto the projecting portions (9b, 19b) extending into the outflow cells of the plugged honeycomb structure is an NO x adsorber catalyst.