High performance insulation materials are required for lunar surface exploration and deep space exploration that experience long-term low temperature environment, and their thermal conductivities are important parameters in thermal design. Guarded Hot Plate (GHP) method is used for measuring a thermal conductivity of thermal insulation material. However, since the conventional GHP method has a constraint on sample size in order to reduce the effect of heat leak, it has always been difficult to measure small-sized specimens’ thermal conductivity. Then, we try to improve the measurement accuracy of thermal conductivity by changing the temperature difference of specimen in the mini GHP (130×130 mm) method.