We have developed a new composite material for hydrogen tanks. The newly developed composite material has high hydrogen gas barrier performance and high durability. The material was prepared by sandwiching Claist^®, a clay-based film, between prepreg sheets of CFRP, and then heat pressing them. The material has a simple structure and higher reliability than the conventional materials that use aluminum or high-density plastics as gas barrier liners. The preliminary test results show that the hydrogen gas barrier performance of this material is better than those of other polymer materials by approximately two orders of magnitude. This corresponds to a leakage of 0.01 % of gas per year from a 50-atm hydrogen tank that is 5 m in length and 1 m in diameter. This material is applicable to lightweight hydrogen tanks for use in aircraft, spacecraft and vehicles.