To make ballast tanks at both sides ventilable thoroughly at the same time by installing a valved communicating port in a vertical partition wall in order to do ventilation by means of supply air from one side vent hole of two ballast tanks at both the port and starboard sides and exhaust air from the other side vent hole.

A vent hole 5a is formed in the upside of each ballast tank 5, and a reversible ventilating fan 21 is installed in this vent hole 5a. At a time when ventilation is carried out after drainage of individual ballast tanks, a valved communicating port 20 of a vertical aprtition wall 4 is opened in advance, and fresh air is fed to the vent hole 5a in the upside of one side ballast tank out of one side of ballast tanks 5 in these ballast tanks at both the port and starboard sides, from a standing or temporary air-feed fan 21. If so, a flow of air is produced in space between these ballast tanks through the opened valved communicating port 20, whereby exhaust air is carried out of the vent hole 5a at the upside of the other side ballast tank, and thus the fresh air is thoroughly filled up simultaneously.