  We show a microscopic derivation of a quantum master equation with counting
terms which describes the electron statistics. A localized spin behaves as a
probe whose precession angle monitors the net electron current by the
magnetic-moment interaction. The probe Hamiltonian is proportional to the
current, and is determined self-consistently for a model of a quantum dot. Then
it turns out that the quantum master equation for the spin-precession contains
the counting terms. As an application, we show the fluctuation theorem for the
electron current.
