We have carried out a de Hass-van Alphen (dHvA) experiment for the recently discovered CePt3Si, which is the first heavy fermion superconductor without inversion symmetry in the tetragonal crystal structure, together with a dHvA experiment for a non-4f reference compound LaPt3Si. As for LaPt3Si, several dHvA branches were observed. Among them, the two main dHvA branches with the dHvA frequency (the cyclotron effective mass) of 1.10 x 10(8) Oe (1.4 m(0)) and 8.41 x 10(7) Oe (1.5 m(0)) were found to be well explained from the FLAPW energy band calculations, corresponding to bands 64- and 63-multiply-connected hole Fermi surfaces, respectively. On the other hand, the dHvA frequencies of 10(7) Oe in CePt3Si are small in magnitude, although the corresponding cyclotron masses of 10-20 m(0) are extremely large.