Provided a LNG ship that is manufactured in a short construction period by assembling a LNG tank on land and mounting the LNG tank in a hold of the ship.A cold insulator and a membrane are affixed on an inner side of a prismatic tank to fabricate a LNG tank, which is mounted in a hold having a double hull structure. In order to prevent the prismatic tank from deforming at the time of the mounting, strength members are welded to an outer surface of the prismatic tank before a thermal insulation work for the purpose of sufficient reinforcement. After the tank is mounted in the hold, the strength members of the prismatic tank are coupled to the inner hull of the ship to integrate the LNG tank and the hull, so that the weight of a liquid cargo is supported by the prismatic tank and the hull together.