Abstract: Marine gas hydrate, which had been considered non-commercial for almost 20~30 years, is proved an important new energy source. The methodology and technology for extraction of methane from deep sea gas hydrate have been solved recently. The US and Japan have made plans to initiate testing production of methane from gas hydrates. The method of depressurization is proven more economic and effective. In this article, we try to make an introduction to the Japanese "Gas hydrate exploration and production program MH21". In addition to the detailed description of the plan, the design of production testing is also discussed in details.