Radiometric ages of detrital zircons in three samples of psammitic schists from the Sanbagawa Belt, Kanto Mountains, were obtained from the ²³⁸U/²⁰⁶Pb ratio and isotopic compositions of Pb using a Sensitive High Resolution Ion MicroProbe (SHRIMP II). Most of the zircon ages cluster around Cretaceous, with a few ages corresponding to older zircons. The origins of the detrital zircons are mainly Cretaceous igneous rocks. The ages of the youngest zircons in samples AM48p, SnbE, and AM29p indicate Late Cretaceous time, and they are 78.8 ± 1.3 Ma, 91.4 ± 1.4 Ma, and 95.3 ±1.5 Ma, respectively. The samples AM48p and AM29p have K-Ar ages of 65.9 ± 1.4 Ma and 82.1 ± 1.8 Ma, respectively. The age difference between the youngest detrital zircon age and white mica K-Ar age is 13 Myr. The Sanbagawa Belt is believed to be a metamorphosed phase of the Chichibu Belt, which is a Middle Jurassic to earliest Cretaceous accretionary complex; however, the results of this study suggest that the protolith of the Sanbagawa belt was accreted in Late Cretaceous, similar to the Shimanto belt that runs parallel to the Sanbagawa and Chichibu belts.