The spin-polarized electron momentum distribution (magnetic Compton profile: MCP) of ferromagnetic CeRh3B2 has been measured using the magnetic Compton scattering technique at 10 K for the [11bar 20] direction. In this compound, the orbital moment dominates the magnetization. An analysis of the MCP reveals that the Ce 4f and Ce 5d spin moments are aligned parallel with a value of -0.68 +/- 0.08 and 0.41 +/- 0.04muB, respectively, while Rh 4d electrons do not carry any significant spin moment. The ratio of orbital to spin moments for Ce 4f electrons, which has been determined from comparison of the present data with recent polarized neutron diffraction data, is rather smaller than that for Ce3+ ions. This result implies strong hybridization of the Ce 4f orbital with the orbitals on the surrounding ions.