  We investigate, within the framework of three generations of neutrinos, the
effects of CP violation in long-baseline neutrino oscillation experiments. We
aim at illuminating the global feature of the interplay between genuine effect
due to the CP violating phase and a fake one due to the earth matter effect. To
this goal, we develop a formalism based on the adiabatic approximation and
perturbative treatment of the matter effect which allows us to obtain
approximate analytic expressions of the oscillation probabilities. We present
an order-of-magnitude estimation and a detailed numerical computation of the
absolute and the relative magnitudes of the CP violations under the mass
hierarchy suggested by the atmospheric neutrino anomaly and the cosmological
dark matter. We find that the genuine CP violating effect is at most \sim 1%,
and the matter effect dominates over the intrinsic CP violation only in a
region of parameters where the oscillation probability of $\nu_\mu\to\nu_e$ is
large.
