PURPOSE: To efficiently pump the cargo oil into a cargo oil tank which is divided into the upper and lower parts by a horizontal bulkhead.

CONSTITUTION: An unloading pump 8 is provided oh the stem side of a tanker where an upper cargo oil tank 5a is partitioned from a lower cargo oil tank 5b by a horizontal bulkhead 16 which is formed approximately parallel to an upper deck 2 and at the height of the water line, and the upper and lower cargo tanks are divided in the longitudinal direction by providing transverse bulkheads 4 in an approximately vertical and transverse manner with the prescribed space in the longitudinal direction. An unloading line 9a which is provided with a suction port for unloading in the lower cargo oil tank 5b and connected to the suction side of the unloading pump 8 is provided, and a first transfer line 17a to transfer the cargo oil from the respective upper cargo oil tanks 5a to the lower cargo oil tank 5b immediately therebelow, and a second transfer line 17b to transfer the cargo oil to the lower cargo oil tank 5b adjacent to the unloading pump 8 side of the lower cargo oil tank 5b are provided.