Torus plasma merging technique that provides high power ion heating by magnetic reconnection is expected to significantly expand the operation scenarios of spherical tokamak (ST) fusion reactors. When two torus plasmas with plasma currents in the same direction are merged, magnetic flux change induces a sheet current in the opposite direction between them. In order to establish ST equilibrium after merging, this opposite current must be reversed quickly. The plasma current rearrangement mechanism during/after plasma merging was investigated in the UTST plasma merging experiment. In the downstream region, a larger electrostatic field than that expected in the steady state is formed, and reversed the parallel electric field along the magnetic field lines. The radial profile of the parallel electric field shows good agreement with the toroidal current density and the soft x-ray emission distribution, indicating that the parallel electric field governs the behavior of electrons in the downstream region and contributes to form the plasma current distribution that sustain the equilibrium of the merged ST.