Three dimensional MHD simulations areexecuted in a full toroidal geometry to clarify the physicalmechanisms of the internal reconnection event (IRE) which is observedin spherical tokamak experiments. The simulation results reproduceseveral main properties of IREs. A notable property in the lineargrowth of modes is the simultaneous excitation of multiple low nmodes, which grow together with similar growth rates. A spontaneousphase alignment mechanism among these excited modes is found in thenon-linear stage of the development, which causes a bulge-likedeformation of the torus and subsequent expulsion of heat energy tothe external region through the occurrence of external reconnection.