[en] The Helium Engineering Demostration Loop (HENDEL) was constructed in March, 1982. The hot gas duct is installed in the Adapter section of HENDEL. The hot gas duct of 15 m in length connects the second-stage He gas heater H₃₂ and the first-stage He gas cooler C₃₁. Thermal performance tests of hot gas duct have been carried out since March, 1982. By measuring temperatures of the pressure tube and internal insulation layer and the heat flux distributions at the surface of the duct, the experimental correlation of the effective thermal conductivity of internal insulation layer was obtained. In this paper, thermal diffusivity of internal insulation layer is estimated by analyzing the transient temperature measurements with a heat transfer computer code, AYER. The following values were obtained: Thermal diffusivity : 1.5x10^-6(m²/s) Specific heat : 1.16(kj/kg.K). (author)