In the global demand for the environment friendly energy, LNG based trade has been increasing steadily, and demand for LNG carriers by customers has become more diversified. Under these circumstances, Mitsubishi Heavy Industries, Ltd. (MHI) has established the building record of Moss type LNG carriers, and also has been carrying out the technical developments, repair, and maintenance work of membrane LNG carriers first started in the 1970s. Thanks to the customer's evaluation for those activities, MHI was awarded the contract for six large membrane ships in 1999, which were the first large sized ships to be built in Japan. These ships were built and delivered by 2004 by MHI. Advanced design and building methods with new technologies were incorporated in the building of these ships which were endorsed by spherical tank technique as well. Accordingly, the most advanced and distinguished, largest class membrane type LNG carriers in the world were completed. Now MHI is continuing work on the development of the next generation LNG carriers to meet the further needs of customers.