  We examine accuracy of theta_{23} determination in future long-baseline (LBL)
nu_{mu} disappearance experiments in the three-flavor mixing scheme of
neutrinos. Despite that the error of sin^2{2theta_{23}} is indeed a few % level
at around the maximal mixing, we show that the error of physics variable
s^2_{23} is large, delta(s^2_{23})/s^2_{23} \simeq 10-20%, depending upon
regions of theta_{23}. The errors are severely affected by the octant
degeneracy of theta_{23}, and delta(s^2_{23}) is largely amplified by the
Jacobian factor relating these two variables in a region near to the maximal
mixing. The errors are also affected by the uncertainty due to unknown value of
theta_{13}; delta(s^2_{23}) is doubled at off maximal in the second octant of
theta_{23} where the effect is largest. To overcome this problem, we discuss
combined analysis with nu_e appearance measurement in LBL experiments, or with
reactor measurement of theta_{13}. For possible relevance of sub-leading
effects even in the next-generation LBL experiments, we give a self-contained
derivation of the survival probability to the next to leading order in s^2_{13}
and Delta m^2_{21} / Delta m^2_{31}.
