  We study the structure of CP violating phases in the seesaw model. We find
that the $3\times 6$ MNS matrix contains six independent phases, three of which
are identified as a Dirac phase and two Majorana phases in the light neutrino
sector while the remaining three arise from the mixing of the light neutrinos
and heavy neutrinos. We show how to determine these phases from physical
observables.
