PURPOSE: An SCR(Selective Catalytic Reduction) reactor of a ship or a marine structure is provided to increase space operability, to omit elements and components which are used for protecting and insulating an external by-pass pipe, and to minimize the pressure loss of an exhaust pipe. CONSTITUTION: An SCR reactor of a ship or a marine structure includes a housing(10) inside which a main exhaust path and a by-pass path. The main exhaust path is placed between the inlet and the outlet of the housing. A catalyst for reaction with an exhaust gas is placed inside the main exhaust path. The by-pass path diverts exhaust gas which flows in the inlet so that the exhaust gas cannot pass through the main exhaust path, and sends the exhaust gas to the outlet. A first path change unit(16a) and a second path changing unit(16b) are respectively installed at the inlet and the outlet so that one of the main exhaust path and the by-pass path can be selectively connected to the inlet and the outlet. The by-pass path is restrictively formed inside a by-pass pipe of which the inlet end and the outlet end are connected to the inlet and the outlet by the first path change unit and the second path changing unit, and the main exhaust path is restrictively formed outside a by-pass pipe.
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