The Nishisonogi unit of the Nagasaki Metamorphic Rocks represents a part of a Late Cretaceous subduction complex exposed in western Kyushu, Japan. We estimate peak metamorphic temperatures using a Raman carbonaceous material (CM) geothermometer on 60 pelitic schists. No systematic regional changes were observed in the mineral assemblage of samples collected over a large area (about 30 × 15 km), which include chlorite ± garnet + white micas + albite + quartz + titanite + CM. However, the estimated peak metamorphic temperature increases structurally upward from 440 to 524 °C, suggesting an inverted thermal gradient.