  If the atmospheric neutrino oscillation amplitude, sin^2 2theta_atm is not
maximal, there is a two fold ambiguity in the neutrino parameter space: sin^2
theta_atm>0.5 or sin^2 theta_atm<0.5. In this article, we study the impact of
this degeneracy, the so-called octant degeneracy, on the T2KK experiment, which
is a proposed extension of the T2K (Tokai-to-Kaimoka) neutrino oscillation
experiment with an additional water cherenkov detector placed in Korea. We find
that the degeneracy between sin^2 theta_atm= 0.40 and 0.60 can be resolved at
the 3sigma level for sin^2 2theta_rct>0.12 (0.08) for the optimal combination
of a 3.0^circ off-axis beam (OAB) at SK (L=295km) and a 0.5^circ OAB at
L=1000km with a far detector of 100kton volume, after 5 years of exposure with
1.0(5.0) time 10^21 POT/year, if the hierarchy is normal. We also study the
influence of the octant degeneracy on the capability of T2KK experiment to
determine the mass hierarchy and the leptonic CP phase. The capability of
rejecting the wrong mass hierarchy grows with increasing sin^2 theta_atm when
the hierarchy is normal, whereas it is rather insensitive to sin^2 theta_atm
for the inverted hierarchy. We also find that the 1sigma allowed region of the
CP phase is not affected significantly even when the octant degeneracy is not
resolved. All our results are obtained for the 22.5 kton Super-Kamiokande as a
near detector and without an anti-neutrino beam.
