  We introduce "CP trajectory diagram in bi-probability space" as a powerful
tool for pictorial representation of the genuine CP and the matter effects in
neutrino oscillations. Existence of the correlated ambiguity in a determination
of CP violating phase \delta and the sign of \Delta m^2_{13} is uncovered.
Principles of tuning beam energy for a given baseline distance are proposed to
resolve the ambiguity and to maximize the CP-odd effect. We finally point out,
quite contrary to what is usually believed, that the ambiguity may be resolved
with 50 % chance in the super-JHF experiment despite its relatively short
baseline of 300 km.
