To provide an opening of two different parts in a single ballast tank, in a liquefied gas ship of independent tank system partitioning the ballast tank into a plurality of parts in a position different from a cross bulkhead of a plurality of freight tanks.

A liquefied gas ship 1 of independent tank system has No.1 to No.4 ballast tanks 3a to 3d partitioned into a plurality of parts in a position different from a cross bulkhead 2B in a lower part of a plurality of No.1 to No.3 freight tanks 2a to 2c, of No.1 to No.4 access spaces 5a to 5d provided in a location of a bulkhead 2B in order to lead to the ballast tanks 3a to 3d from an upper deck, in the No.2, No.3 access spaces 5b, 5c except both ends, horizontal passages 6a, 6b leading to either one of cross bulkheads 3B sandwiching these spaces are provided, openings 7, 8 of different two parts are arranged in a suitable part of the No.1 to No.4 access spaces 5a to 5d and the horizontal passages 6a, 6b.