PURPOSE: To prevent water from splashing on an engine and avoid exposing a propelling device to the exterior and thereby prevent various troubles by a method wherein the engine is mounted at a rear part of a surfboard through a mount box having a predetermined height, and the propelling device is installed in the box.

CONSTITUTION: An engine chamber 12 is formed at a ridge part 6 of the rear part of the surfboard 1, and the engine 14 is mounted on a square engine mount box 13 placed at the bottom of the chamber 12, with a crankshaft 21 for the engine 14 disposed vertically. The box 13 has the predetermined height, and an waterproofing cover 15 is fitted to the peripheral edge of the box 13 so as to cover the periphery of the engine 14 up to a height close the upper end of the engine 14. A water jet propulsion device 43 is placed in the interior of the box 13, with a impeller 46 connected to a lower end part of the crankshaft 21. Nu- meral 47 represents a water intake port and numeral 48 represents a jet nozzle.