Technigaz type membrane tank of an LNG carrier is composed of a membrane and a thermal insulation structure. To assure the safety and reliability of a tank, it is important to make quality control in the installation process of tank members so as to check that they are properly installed to show the required functions. In order to confirm and improve the tank inspection method proposed by Technigaz, the licensor, NKK has applied various inspection methods to the LNG mock-up tank built as its Tsu Works. As a result, NKK has not only succeeded in establishing the assurance system of the tank to be used in the construction of LNG carrier, but also has confirmed the perfect safety of the mock-up LNG tank by the system.