We report on a study of thermal Hall conductivity κxy in the superconducting state of CeCoIn5. The scaling relation and the density of states of the delocalized quasiparticles, both obtained from κxy, are consistent with d-wave superconducting symmetry. The onset of superconductivity is accompanied by a steep increase in the thermal Hall angle, pointing to a striking enhancement in the quasiparticle mean free path. This enhancement is drastically suppressed in a very weak magnetic field. These results highlight that CeCoIn5 is unique among superconductors. A small Fermi energy, a large superconducting gap, a short coherence length, and a long mean free path all indicate that CeCoIn5 is clearly in the superclean regime (εF Δ ξ), in which a peculiar vortex state is expected. © 2005 The American Physical Society.