To provide an exhaust structure of a jet propulsion type small planing boat capable of restraining a repelling sound by pulsation of exhaust gas, and further improving cooling efficiency of a water muffler.

This exhaust structure of the small planing boat is propelled by reaction by jetting water pressurized-accelerated by a water jet pump P from a rear jet nozzle, and exhausts exhaust gas from an engine E for driving the water jet pump P outside a ship via the water muffler 31 formed as a double wall structure, and constituted so as to make cooling water to flow between walls (a water jacket 31w). The water muffler 31 is constituted so as to discharge cooling water from the upper side by taking in the cooling water flowing to the water jacket 31w from the lower side.