There are many uncertain factors involved in the cosmic ray experiments (on atmospheric neutrinos) which seek to confirm the existence of neutrino oscillations with definite oscillation parameters. For the purpose, it is desirable to select those physical events in which uncertain factors are involved to a minimum extent and to examine them in a rigorous manner. In the present paper we consider neutrino events due to quasielastic scattering (QEL) as the most reliable events among various candidate events to be analyzed, and have carried out the first step of an L/E analysis which aims to confirm the survival probability with a Numerical Computer Experiment. The most important factor in the survival probability is L ν /E ν , but this cannot be measured for such neutral particles. Instead, L µ /E µ is utilized in the L/E analysis, where L ν , L µ , E ν and E µ denote the flight path lengths of the incident neutrinos, those of the emitted leptons, the energies of the incident neutrinos and those of the emitted leptons, respectively. According to our Computer Numerical Experiment, the relation of L ν /E ν ≈ L µ /E µ doesn't hold. In subsequent papers, we show the results on an L/E analysis with the Computer Numerical Experiment based on our results obtained in the present paper.