[en] The High Temperature Engineering Test Reactor (HTTR) is the first High Temperature Gas-cooled Reactor (HTGR) built at the Oarai Research and Development Center of JAEA with thermal power of 30 MW and the maximum reactor outlet coolant temperature of 950degC. The HTTR achieved the first criticality in 1998, the reactor outlet coolant temperature of 950degC in 2004, and 30 days continuous operation in 2007. Since 2002, safety demonstration tests including reactivity insertion tests and coolant flow reduction tests have been conducted to show inherent safety features of the HTGRs by using the HTTR. This report describes full scope of the future feasible test plan mainly using the HTTR. The test items cover fuel performance and radionuclide transport, core physics, reactor thermal hydraulics and plant dynamics, and reactor operations, maintenance, control, etc. The test results will be utilized for realization of Japan's commercial Very High Temperature Reactor (VHTR) system, GTHTR300C. (author)