  Magnetism and superconductivity of new heavy fermion compounds CeTIn$_5$
(T=Co, Rh and Ir) are investigated by applying fluctuation exchange
approximation to an orbital degenerate Hubbard model. The superconducting phase
with $d_{x^2-y^2}$-symmetry is found to appear next to the antiferromagnetic
phase with increasing the orbital splitting energy. The present theory suggests
that the orbital splitting energy plays a key role of controlling parameter for
the quantum phase transitions in the heavy fermion system.
