Dynamics of a relaxation phenomenon observed in Spherical Tokamak experiments, which is called Internal Reconnection Event (IRE), is studied by magnetohydrodynamic computer simulation. Coupling between the torus magnetic field and the external field induced by nonlinear growth of pressure driven instabilities, and resultant formation of tunnel-like plasma jet flow are found to play crucial roles in the mechanism of quick expulsion of the plasma heat energy into the external region.