  In the SK analysis of the neutrino events for [Fully Contained Events] and
[Partially Contained Events] on their zenith angle distribution, it is assumed
that the zenith angle of the incident neutrino is the same as that of the
detected charged lepton. In the present paper, we examine the validity of [the
SK assumption on the direction] of the incident neutrinos. Concretely speaking,
we analyze muon-like events due to QEL. For the purpose, we develop [Time
Sequential Monte Carlo Simulation] to extract the conclusion on the validity of
the SK assumption. In our [Time Sequential Simulation], we simulate every
physical process concerned as exactly as possible without any approximation.
  From the comparison between the zenith angle distributon of the emitted muons
under [the SK assumption on the direction] and the corresponding one obtained
under our [Time Sequential Simulation], it is concluded that the measurement of
the direction of the incident neutrino for the neutrino events occurring inside
the detector in the SK analysis turns out to be unreliable, which holds
irrespective of the existence and/or non-existence of the neutrino oscillation.

High Energy Physics - Experiment;Astrophysics;High Energy Physics - Phenomenology