To easily perform the work of extracting engine oil by providing a removable drain plug on the bottom part of an oil tank, forming an opening part on a hull so as to be situated under the drain plug, and mounting an attachable and detachable lid body on the opening part.

An engine 14 is loaded within an engine room 15 formed by bonding a deck 4 to a hull, a front oil tank 600 and a rear oil tank 70 for storing the engine oil to be supplied to the engine 14 are arranged on the bottom part of the engine room 16, and a drain plug 88 is attachably and detachably provided on the bottom part of the rear oil tank 70. An attachable and detachable lid body 101 is mounted on an opening part 10 formed on the hull so as to be situated under the drain plug 88 and a cover 69 by a bolt 102. Thus, the lid body 101 blocking the opening part 100 of the hull is removed, whereby the drain plug 88 can be easily removed from the opening part 100, and the work of extracting the engine oil from the rear oil tank 70 can be easily performed.