Summary: Quantum mechanical perturbation theory is formulated for a quasidegenerate system. The equation of des Cloizeaux and Brandow, which has been a basic equation for determining the usual non-Hermitian effective interaction in the degenerate perturbation theory, is generalized to the quasidegenerate case. It is proved that the generalized equation can be solved exactly and the usual non-Hermitian effective interaction can be given in an explicit form even though unperturbed energies are not degenerate. The structure of Hermitian form is also discussed.