We have succeeded in observing the de Haas-van Alphen effect in LaOs4P12 which is a superconductor with the transition temperature T-C = 2.0 K and a reference compound for PrOs4P12. We have found the multiply-connected and almost spherical Fermi surfaces whose cyclotron effective masses are ranging 1.1-4.7m(0). The Fermi surface topology is close to that of PrOs4P12, indicating the localized nature of 4f-electrons in PrOs4P12. From the comparison with LaOs4P12, the large mass enhancement in PrOs4P12 has been clarified.