  We examined the effect of an imaginary part of the ladder approximation
Schwinger-Dyson equation. We show the imaginary part enhances the effect of the
first order transition, and affects a tricritical point. In particular, a
chemical potential at a tricritical point is moved about 200(MeV). Thus, one
should not ignore the imaginary part. On the other hand, since an imaginary
part is small away from a tricritical point, one should be able to ignore an
imaginary part. In addition, we also examined the contribution of the wave
function renormalization constant.
