  As a possible approach to the neutrino oscillation on the basis of quantum
field theory, the expectation values of the flavor-neutrino currents are
investigated by employing the finite-time transition matrix in the interaction
representation. Such expectation values give us in the simplest form a possible
way of treating the neutrino oscillation without recourse to any one
flavor-neutrino states. The present paper is devoted to presenting the
formulation and the main structures of the relevant expectation values.
