A large amount of undeveloped groundwater may still be left in fractured rocks of rocky mountainous areas. Points with high potential for groundwater development may be found near fault zones. The indexes ²¹⁴Bi and ²¹⁴Bi/²⁰⁸Tl, measured by γ-ray radioactivity prospecting, are useful for detecting buried faults. We developed a radioactivity prospecting device with a three-layered plastic scintillator (PS) detector. This device is lighter weight, easier to use, and less expensive than conventional NaI detectors. A simple method for discriminating γ-rays of ¹³⁷Cs, ⁴⁰K, ²¹⁴Bi, and ²⁰⁸Tl on a spectrum is proposed. We performed experiments that showed that the PS can be used for radioactivity prospecting.