An exhaust gas cleaning device is provided for an outboard motor unit having an engine provided with a pair of cylinder rows arranged so as to provide a V-shape and a crank shaft arranged substantially in a perpendicular direction in a using state and having an exhaust expansion chamber on a side of a cylinder head of the engine. In the improvement, a catalyst is disposed below the engine and inside a space having substantially a triangle shape, in a plan view, defined by a central line of one of the cylinder rows, a central line of another one of the cylinder rows and a central line of the expansion exhaust chamber. The catalyst is also disposed, in an arrangement of the cylinder rows providing a V-shape having an angle of less than 90.degree., below the engine and inside a space having substantially a triangle shape, in a plan view, defined by an inner wall section of one of the cylinder rows, an inner wall section of another one of the cylinder rows and a wall section, facing the V-shape section of the cylinder rows, of the expansion exhaust chamber.