A one‐dimensional electrostatic particle simulation model was constructed to investigate an evolution of the parallel electrostatic potential in the presence of a plasma flow along the mirror field. It is shown that a steady‐state, large‐scale dc potential structure is created as a result of a continuous plasma flow injection, and that the resultant potential difference increases in proportion to the ion bulk flow energy. In the magnetospheric environment, the potential difference is found to be able to exceed 2.5 kV.