We have investigated the Fermi-surface (FS) properties in the Pr-based filled skutterudite PrOs4P12 and its reference compound LaOs4P12 by means of de Haas-van Alphen experiments and the band-structure calculations. The topology of FS in PrOs4P12 is close to that in the reference compound LaOs4P12, indicating a localized nature of 4f electrons in PrOs4P12. Whereas the localized nature of 4f electrons, we have confirmed a highly enhanced cyclotron effective mass of up to 18m(0) in PrOs4P12, which is enhanced about 3.8 times compared to that in LaOs4P12. No nesting property with q=(1,0,0) in the FS of PrOs4P12 has been confirmed in contrast to good nesting properties in PrFe4P12 and PrRu4P12 which exhibit unusual ordered states at low temperatures. A role of both the 4f electron's contributions and the FS nesting property for unusual phase transitions are discussed.