We derived the thermal structure of the Kuju region of volcanic activity in central Kyushu, Japan, to assess the potential existence of a supercritical geothermal system. Extrapolation of existing well-logging data suggests the presence of a high-temperature region beneath the area between the Hatchobaru geothermal power plant and Kuju-Iwoyama Volcano, which closely overlaps a low-resistivity body estimated from magnetotelluric data. Based on estimated temperatures exceeding 380 °C at the relatively shallow depth of -3.3 km above sea level, and considering evidence for magmatic fluid transport, this high-temperature region is a potential target for a supercritical geothermal reservoir.