The temporal dynamics of a plasmoid is numerically studied on the basis of the plasmoid that is in due course formed when magnetic reconnection is coupled to anomalous resistivity. No external agency is imposed, so that the temporal behavior of the plasmoid, defined by three neutral points, X1, O, and X2 (X12), is completely determined by how magnetic reconnection proceeds at the neutral points. It is shown that the plasmoid cannot stand stationarily but is likely to propagate through between the oppositely directed magnetic fields. When the plasmoid moves in the positive x direction, anomalous resistivity tends to be more enhanced near the X1 point than near the X2 point so that the reconnection rate at X1 exceeds that at X2; the magnetic body force resulting from the asymmetric fast reconnection process further accelerates the plasmoid. The plasmoid finally attains a propagation speed almost comparable with the Alfv¿n velocity.