To enable carbon adhered to an exhaust control valve to be positively removed when the exhaust control valve is operated to be cleaned in order to remove the carbon adhered to the exhaust control valve by preventing the riding quality on a boat, or the starting ability of an internal combustion engine from being lowered, or by preventing the accelerating ability from being inhibited.

When the engine speed of an internal combustion engine 13 at the time when a boat is shifted to a planing state where the boat is accelerated and high speed propelled to plane on the water surface, is expressed by 'planing shift engine speed R4', an exhaust control valve 72 is forcedly operated in interlocked with a driving means 79 by controlling the driving means 79 at a specified engine speed below the 'planing shift engine speed R4' by means of a driving control means 82. Thus, the exhaust control valve 72 is operated to be cleaned.