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\section{Introduction}
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The evidence of neutrino oscillation in the atmospheric neutrinos discovered by 
Super-Kamiokande (SK)~\cite{SKatm} is being confirmed by the first 
accelerator-based long baseline (LBL) oscillation experiment, K2K~\cite{K2K}.
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Also, implications of neutrino oscillation in the solar neutrino~\cite{Solar}
is now confirmed by a reactor-based LBL experiment, KamLAND~\cite{KamLAND}.
Existence of finite neutrino masses and large flavor mixing becomes almost
unambiguous. They are the first observations which are contrary to the standard
model.

The neutrinos have anomalously small masses compared with quarks and other
leptons. The See-Saw mechanism try to explain the smallness by introducing 
right-handed heavy Majorana neutrino \cite{SeeSaw}.
The large mixing found in the neutrino sector is very much different
from the small mixing in the quark sector.
Grand Unified Theories, which describes quarks and leptons in a unified manner
at ultra-high energy scale, relate the mixings of quark and lepton sectors.
Precise knowledge on these neutrino properties would provide
a big hint of the physics beyond the standard model.


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Next step in LBL experiments along this direction is to establish (or refute)
the framework of 3-flavor mixing.
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One of the most important things for that purpose is to discover the 
only remaining oscillation mode $\nu_\mu \rightarrow \nu_e$ or finite mixing angle
$\theta_{13}$. The mixing angle is known to be much smaller than the other two
mixing angle~\cite{the13}. Discovery and precise measurement of $\theta_{13}$ would provide a
key to explore the physics at high energy scale.
It is also important to measure oscillation parameters precisely by checking the
spectrum shape after oscillation. Deviation from the predicted oscillation
pattern would imply non-standard model physics, such as extra dimension.
Furthermore, firm confirmation of $\nu_\mu\rightarrow \nu_\tau$ oscillation by 
(1)direct observation of $\nu_\tau$  or (2) observation of neutral current 
interactions are also important. This would provide a constraint
on sterile neutrino.
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Once the finite $\theta_{13}$ is found, search for the CP violation in
the lepton sector becomes realistic. Since the large mixing angle region
is found to be the solution of the solar neutrino problem, the
expected size of the CP asymmetry is within the reach of the next generation LBL 
experiments  provided that $\theta_{13}$ is not extremely small.
Discovery of the CP violation in the lepton sector would be a big
step to understand matter anti-matter asymmetry in the universe.

One or two order of magnitude higher intensity neutrino beam
than current on-going experiment is necessary to achieve the
purposes. Several high statistics LBL experiments with
conventional horn-focused $\nu_\mu$ beam produced by a MW-class proton
accelerator are being proposed. Such attempts are recently called ``superbeam''
experiments, when it is contrasted from LBL experiments with neutrino factory.
In my presentation, the proposed superbeam experiments and their physics
potential are summarized.
