In the Japanese hydrogen project, World Energy Network (WE-NET) for realizing reusable clean energy systems using hydrogen, conceptual designs of large-mass liquid hydrogen storage systems for ground and transportation tanks have been studied. For completing such large storage systems, of which scales would reach to that of commercialized liquid natural gas (LNG) facilities, this study has concluded that thermal insulation structures satisfying large-tank requirements should be developed. In order to evaluate thermal performances of various devised insulation structures between 20 and 300K, we have developed a large experimental apparatus with a double-guarded flat plate boil-off calorimeter method. This apparatus can test various kinds of specimens with allowable dimensions; diameter 1.2m, up to thickness 0.3m, to provide the thermal data needed for designing a full-scale storage tank. This paper describes the abstract of the devised large-mass liquid hydrogen storage tank, the experimental errors of the experimental apparatus to be considered, and the results of the background and heater tests for evaluating its performance. The background test result showed that heat transfer to the measuring vessel was 0.1W, small enough to satisfy apparatus performance requirements.