We have carried out de Haas-van Alphen (dHvA) experiments on a ferromagnet CeRh3B2 with an extremely high Curie temperature T-C similar or equal to 120 K and a non-4f reference compound LaRh3B2. The dHvA data of LaRh3B2 are well explained by the results of energy band calculations. The topology of the Fermi surfaces in CeRh3B2 is found to be very similar to that of LaRh3B2, possessing wavy but flat Fermi surfaces in the basal plane. Observation of a quasi-one-dimensional electronic state is the first such case in a rare earth compound.