  The dissipation process in two dimensional BCS superconductor due to the
quasiparticles in the core of moving vortex is studied from a quantum
mechanical point of view, especially paying attention to the spectral flow of
the bound state energies in the vortex core. In order to clarify the nature of
the spectral flow, we performed a numerical study of a finite system and, by
extending the analysis to include the effect of impurities, we discuss the
quantum mechanical origin of the dissipation in the vortex core.
