  If the lepton mixing angle theta_{23} is not maximal, there arises a problem
of ambiguity in determining theta_{23} due to the existence of two degenerate
solutions, one in the first and the other in the second octant. We discuss an
experimental strategy for resolving the theta_{23} octant degeneracy by
combining reactor measurement of theta_{13} with accelerator nu_{mu}
disappearance and nu_{e} appearance experiments. The robustness of the
theta_{23} degeneracy and the difficulty in lifting it only by accelerator
experiments with conventional nu_{mu} (and nu_{mu}-bar) beam are demonstrated
by analytical and numerical treatments. Our method offers a way to overcome the
difficulty and can resolve the degeneracy between solutions sin^2 theta_{23} =
0.4 and sin^2 theta_{23} = 0.6 if sin^2 (2 theta_{13}) \gsim 0.05 at 95% CL by
assuming the T2K phase II experiment and a reactor measurement with an exposure
of 10 GW.kt.yr. The dependence of the resolving power of the octant degeneracy
on the systematic errors of reactor experiments is also examined.
