  We explore as clearly as possible the features of neutrino oscillation which
are relevant for measurements of the CP violating Kobayashi-Maskawa phase delta
and the sign of \Delta m^2_{13}. We focus on the so called low-energy option
and discuss principles for optimizing experimental parameters to measure these
two quantities simultaneously. Toward the goal, we first formulate a method for
obtaining a bird-eye view of the phenomenon of neutrino oscillation by
introducing a new powerful tool called the ``CP trajectory diagram in
bi-probability space''. It allows us to represent pictorially the three effects
separately in a single diagram; effect from genuine CP violation due to the sin
delta term, effect from the CP conserving cos delta term, and the fake CP
violating effect due to earth matter. By using the CP trajectory diagram we
observe that there is a two-fold ambiguity in the determination of delta which
is related with the sign of Delta m^2_{13}. We then address the question of
what are the promising options for conceptual design of experiments at low
energies which looks for CP violation and at the same time would resolve the
two-fold ambiguity. We point out that a version with distance of about 700 km,
with a megaton class water Cherenkov detector gives an optimal design which
allows simultaneous determination of delta and the sign of Delta m^2_{13} {\it
in situ}. We also point out that there is a possibility that the similar {\it
in situ} measurement of both quantities can be done at the Phase II of JHF
experiment with much shorter baseline, under the assumption of nature's kind
setting of delta to the region of sin{delta} \cdot Delta m^2_{13} < 0. A
technique of running at high (sim 1 GeV) and low (sim 0.5 GeV) beam energies is
proposed as a method for better identification of delta.
