a first oxidation catalytic converter disposed in an exhaust system of the engine, for reducing nitrogen monoxide in exhaust emission by an oxidation reaction;

a reduction catalytic converter disposed in said exhaust system of the engine, for reductively purifying nitrogen oxides in exhaust emission with a reducing agent;

a second oxidation catalytic converter for ammonia-slip disposed on a latter stage of said reduction catalytic converter in said exhaust system;

an injection nozzle disposed within an exhaust passage of said exhaust system, for injection-supplying the reducing agent to an upstream side of a flow of exhaust emission with respect to the reduction catalytic converter within an exhaust passage of said exhaust system from a downstream side of the flow of exhaust emission with respect to said first oxidation catalytic converter; and

a reducing agent supply device which supplies the reducing agent to the injection nozzle,

wherein the exhaust passage is provided, in an inner wall of a lower portion thereof, with a recess portion by concaving a given site to which at least the reducing agent injection-supplied via the injection nozzle is attached, and

wherein the recess portion has a bottom wall thereof configured to be detachably assembled.