PURPOSE: To provide a double-hull type crude oil tanker prevented from the damage of the side membrane and bottom membrane of a crude oil tank at the time of an accident such as a collision or grounding while enabling the completely separated state of ballast water and crude oil at the time of navigating in the empty state so as to have the same effect as a separate ballast tank and moreover dispense with the dangerous internal washing of the crude oil tank for the loading of ballast water.

CONSTITUTION: In a crude oil tank section 31, a double bottom plate 36 is fitted through a space 24 of specified dimension to a bottom shell plate 22, and a front oil box body 32a and a rear oil box body 32b of box shape are formed into bag shape by synthetic resin made side membranes 35 and synthetic resin made bottom membranes 37 respectively facing bulkheads 26 and the shell plate 22 through the specified dimension. The front and rear oil box bodies 32a, 32b have built-in oil tanks 33a, 33b with the upper edges rigidly fixed to the lower face of an upper deck 27, and the inside of each oil tank is partitioned by longitudinal partitioning membranes 38 and horizontal partitioning membrane 39 so as to form collective body of six crude oil tanks 34 made of synthetic resin.