A numerical model is developed to evaluate the decay time of plasma current during the disruption following intense impurity injections. Our model is based on the power balance equation between joule heating and impurity radiation as a way to evaluate the electron temperature and charge state after the thermal quench. The model is applied to the experimental results of massive N 2 gas injections in JFT-2M, which simulate the "ingress-ofcoolant event" (ICE) as well as disruption mitigations in ITER. It is confirmed that the model can reproduce the experiments reasonably well.