We have succeeded in observing the de Haas-van Alphen effect in PrOs 4P12 which shows no anomaly down to 50 mK in contrast to PrFe4P12 and PrRu4P12. The Fermi surface (FS) topology is reasonably explained by the energy band structure calculation based on a full potential linearized augmented-plane-wave method within the local-density- approximation. Whereas the localized nature of 4f-electrons, we have confirmed a highly enhanced cyclotron effective mass up to 18m0 in PrOs 4P12, which is enhanced 3.8 times compared to that of LaOs4P12. No nesting property with q = (1, 0, 0) in the FS of PrOs4P12 has been confirmed, which suggests both the 4f-electron's contribution and the FS nesting property play the role for the phase transitions in PrFe4P12 and PrRu4P 12. ©2008 The Physical Society of Japan.