  The drift term appearing in an anaylitically obtained kinetic equation for a
point vortex system is evidenced numerically. It is revealed that the local
temperature in a region where the vortices are frequently transported by the
diffusion and the drift terms characterizes system temperature and its sign is
definitely negative. Simulation results clearly show a ransport process of the
vortices by the diffusion term (outside the clumps) and the drift term (inside
the clumps), which gives a key mechanism of the self-organization, i.e.,
condensation of the same-sign vortices.
