The Upper Cretaceous Nakaminato Group, which contains the Chikko, Hiraiso, and Isoai formations in ascending order, crops out along the Pacific coast of Ibaraki Prefecture, central Japan. This group is composed mainly of sandstone, siltstone, and sandstone-siltstone alternations, with intercalated conglomerate layers at several levels. The siltstone of the Hiraiso and Isoai formations has yielded ammonites and inoceramid bivalves indicating a middle Campanian to Maastrichtian age. Some conglomerate layers in the Isoai Formation reach 1 m in thickness and mostly consist of pebbles and cobbles of rhyolite, dacite, chert, siliceous siltstone, siltstone, sandstone, and hornfels. We obtained late Paleozoic to Late Jurassic radiolarians from pebbles of argillaceous rock and chert from four levels of the conglomerate layers within the Isoai Formation. We describe the radiolarians systematically herein. The probable provenance of the radiolarian-bearing pebbles is interpreted as the Ashio and Yamizo terranes, which consist of Late Jurassic to Early Cretaceous accretionary complexes. We propose that there were two denudation stages of the accretionary complexes in the Kanto District, stages α (Barremian–) and β (Campanian–).