The nuclear heat utilization systems connected to high-temperature gas-cooled reactors (HTGRs) will be designed on the basis of non-nuclear-grade standards in terms of easier entry for the chemical plant companies and the construction economics of the systems. Therefore, it is necessary that the reactor operations can be continued even if abnormal events occur in the systems. The Japan Atomic Energy Agency has developed a calculation code to evaluate the absorption of thermal-load fluctuations by the reactors when the reactor operations are continued after such events, and has improved the code based on the high-temperature engineering test reactor (HTTR) operating data. However, there were insufficient data on the transient temperature behavior of the metallic components and the graphite core support structures corresponding to the fluctuation of the reactor inlet coolant temperature for further improvement of the code. Thus, nuclear heat supply fluctuation tests with the HTTR were carried out in non-nuclear heating operation to focus on the thermal effect. In the tests, the coolant helium gas temperature was heated to 120°C by the compression heat of the gas circulators in the HTTR, and a sufficiently large fluctuation of 17°C for the reactor inlet coolant was achieved by devising a new test procedure under the ideal condition without the effect of the nuclear power. Then, the temperature responses of the metallic components and the graphite core support structures were investigated. The test results adequately showed as predicted that the temperature responses of the metallic components are faster than those of the graphite blocks, and the mechanism of the thermal-load fluctuation absorption by the metallic components was clarified.