  A simple model to reproduce strong enhancement of dielectric response near
the morphotropic phase boundary (MPB) is proposed. This model consists of
long-range dipole-dipole interaction and compositional chemical disorder
incorporated by the variation in lengths of dipole moments. By applying Monte
Carlo simulation, we show that there appears a ferroelectric boundary phase
between two types of antiferroelectric phases at an optimal strength of
randomness. In the boundary phase, ferroelectric domain becomes remarkably
large and flexible to external electric fields, leading to huge dielectric
response. This observation indicates that huge dielectric response near the MPB
originates from {\it local} polarization rotation under suppressed anisotropy
by phase competition.
