A honeycomb catalyst body (100, 101) comprising: a plugged honeycomb structure having porous partition walls defining a plurality of cells extending from an inflow side end face to an outflow side end face 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) projecting so as to extend from the partition walls into inflow cells being the cells provided with the outflow side plugged portions and into outflow cells being the cells provided with the inflow side plugged portions and being 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 each of the partition walls of the plugged honeycomb structure is from 40% to 70%, a thickness of each of the projecting portions (9, 19) is from 30 to 140% of a thickness of each of the partition walls, an amount of the catalyst loaded onto the partition walls in a cross section perpendicular 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 an amount of the catalyst to be loaded onto the partition walls is smaller than an amount of the catalyst to be loaded onto the projecting portions (9, 19), the catalyst loaded onto the projecting portions (9a, 19a) extending into the inflow cells of the plugged honeycomb structure is a selective reduction catalyst, and the catalyst loaded onto the projecting portions (9b, 19b) extending into the outflow cells of the plugged honeycomb structure is an ammonia slip preventing catalyst. a plugged honeycomb structure having porous partition walls defining a plurality of cells extending from an inflow side end face to an outflow side end face 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) projecting so as to extend from the partition walls into inflow cells being the cells provided with the outflow side plugged portions and into outflow cells being the cells provided with the inflow side plugged portions and being 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 each of the partition walls of the plugged honeycomb structure is from 40% to 70%, a thickness of each of the projecting portions (9, 19) is from 30 to 140% of a thickness of each of the partition walls, an amount of the catalyst loaded onto the partition walls in a cross section perpendicular 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 an amount of the catalyst to be loaded onto the partition walls is smaller than an amount of the catalyst to be loaded onto the projecting portions (9, 19), the catalyst loaded onto the projecting portions (9a, 19a) extending into the inflow cells of the plugged honeycomb structure is a selective reduction catalyst, and the catalyst loaded onto the projecting portions (9b, 19b) extending into the outflow cells of the plugged honeycomb structure is an ammonia slip preventing catalyst.