A numerical simulation is performed to investigate the nonlinear phase of the collisionless tearing mode instability. The results are found consistent with the expectation from the theory by Galeev, Coroniti, and Ashour-Abdalla, who predicted the existence of an explosive phase of the instability caused by the magnetization of particles by the perturbed component of the magnetic field normal to the neutral sheet. Since electrostatic effects on the evolution of the instability are neglected a priori in the present numerical simulation, we do not obtain the final answer. But there seems to be a good possibility of the explosive evolution of the tearing mode instability in its nonlinear stage. It is argued that for a sufficiently long-wavelength perturbation the nonlinear explosive evolution can become faster than the nonlinear coalescence mode. It is further noted that the particles are heated adiabatically within the magnetic islands by the one-dimensional compression process.