PURPOSE: To facilitate cleaning work as well as to enhance strength and rigidity of a hull by forming the corner parts between bulk heads and the bottom plating into duct constructions, which are in communication with the side tanks and are isolated in a water-tight fashion from the crude oil tanks.

CONSTITUTION: A duct part 3 is formed at the corner between the bulkhead 6 and the bottom plating 7 of an oil tanker, by combining a lowermost deflection-preventive crossbeam 1a and a bottom crossbeam 2a in the adjacency of the bulkhead. This duct part 3 is in communication with both side tanks 4 but separated from the crude oil tanks 5a, 5b watertightly. According to this construction, the corner part, which has been the most difficult area to clean in the cleaning work of curde oil tanks 5a, 5b of conventional type due to the deflection-preventive crossbeam 1 and the bottom crossbeam 2, becomes easy to clean.