A ceramic honeycomb catalyst includes a thin-walled honeycomb structural body (10) and a catalytic substance carried by the honeycomb structural body (10). The honeycomb structural body (10) has a number of longitudinally extending flow passages (13) defined by an outer peripheral wall (11) and partition walls (12) with a reduced thickness (t). The honeycomb structural body (10) satisfies particular relationships between the partition wall thickness (t) and the open frontal area (OFA) or bulk density (G). Notwithstanding the thin-walled partition walls, the honeycomb structural body (10) has practically satisfactory compressive strength characteristics. The catalyst comprising the honeycomb structural body (10) has reduced pressure loss and heat capacity.