Engine exhaust emission control system in outboard engine system

An exhaust gas emission control system for a 4-cycle outboard engine (E) is disclosed. The engine (E) comprises a mounting member (10), an engine block (14) mounted on the mounting member (10), an extension housing (11) coupled to the mounting member (10) and extending downward therefrom; and an engine oil pan (15) mounted under the mounting member (10) within the extension housing (11). The oil pan (15) has a recess formed therein in the lengthwise direction. An exhaust gas expansion chamber is formed in the extension housing (11) for receiving exhaust gas from the exhaust manifold of the engine (E). The engine exhaust emission control system comprises a catalyst assembly (C) positioned in the exhaust gas expansion chamber, the catalyst assembly having a catalyst case (55) aligned with the oil pan in the lengthwise direction thereof. At least a portion of the catalyst case is disposed in the recess in the oil pan (15).