[en] Coaxial double walled piping is planned to be used for a primary cooling system piping of the Very High Temperature Gas-cooled Reactor (VHTR) of JAERI. The piping consists of an outer pressure pipe for the reactor inlet gas flow and an inner pipe with internal insulations for the reactor outlet gas flow. The internal insulations are designed to consist of two layers; metal insulation is in the extremely inside for the higher temperature gas and fibrous insulation is between the inner pipe and the metal insulation. The thermal characteristics of inner pipe with two insulation layers are necessary for the designing of the primary cooling system piping. Authors performed thermal characteristics tests by using Kawasaki's helium test loop (KH-200), which has two hot gas ducts test sections of 267.4 mm diameter and 5,000 mm length with simulated two layers insulation. The one is installed in a horizontal position and the other is in a vertical position. The tests were conducted at the temperature of 500 to 1,000 deg. C, the pressure of 20 and 40 kg/cm²G, and the flow rate of 100 and 200 g/s. The distributions of temperatures and heat flux on the surface of the ducts are confirmed to be within an allowable range. The test results were analyzed, and useful design data of the metal insulation and the fibrous insulation were obtained. (author)