PURPOSE: To assure a wide area on the surface of a deck and prevent the center of gravity of a boat from being shifted due to the fluctuation in the weight of fuel, by placing an inboard engine room behind the center of gravity of the boat, providing a flat deck surface at a front portion and installing a fuel tank near the center of gravity under the deck surface.

CONSTITUTION: An engine room 8, which extends upward to higher than the surface 5 of a deck, is provided behind the center of gravity G of a boat. The surface 5 of the deck in front of the engine room 8 is made flat. A fuel tank 16 is placed near the center of gravity G of the boat under the flat surface 5 of the deck. Since the main deck surface 5 which is flat and extensive is provided in front of the engine room 8, large working space and loading space are assured. Since the fluctuation in the center of gravity G due to that in the quantity of fuel is low, the shift of the angle of trim is so small that the influence upon navigation performance is reduced.