The honeycomb catalyst structure to be used for exhaust gas of a marine engine includes an SCR catalyst supported on a metal honeycomb unit having a shape divided by cell walls into a plurality of cells extending from one end face to the other end face. The SCR catalyst contains TiO2 alone, or contains TiO2 as a principal component and any one type or two or more types of V2O5, WO3, MoO3, SiO2, and Al2O3 in combination. The cell density of the plurality cells is 100 to 500 cpsi, and the slope of a straight line represented by a relationship between the flow velocity and the pressure loss of the exhaust gas passing through the cells is 30 or more and 180 or less.