In 1983, four paragonimiasis cases were diagnosed in people who had consumed uncooked freshwater crabs captured from the Yourou River in Otaki-machi, Chiba Prefecture, Japan. Although a field survey conducted about 30 years ago demonstrated that the diploid form of Paragonimus westermani was extensively distributed in Otaki-machi, its prevalence in other areas was not well examined. In this study, we examined the infection status of Paragonimus metacercariae in the Japanese freshwater crab Geothelphusa dehaani collected from the Kobado area of Katsuura City in Chiba Prefecture from 2011 to 2014. Our findings indicated that the prevalence was 82.1% in 2011 and reached close to 100% between 2012 and 2014. Additionally, the average rate of metacercariae infection in freshwater crabs for four years was 94.3%, with the number of metacercariae per infected crab ranging from 1 to 329 (mean: 27.8). The lung flukes examined were identified as the diploid form of P. westermani based on both the morphological features of the metacercariae and molecular analyses of the nuclear ribosomal internal transcribed spacer 2 region and mitochondrial 16S rRNA genes. These results suggested that the natural life cycle of the diploid form of P. westermani remained well-preserved in Chiba Prefecture. In addition, the Kobado area was added as a new prevalent locality of this lung fluke in Japan.