For understanding of the plug potential generation in a tandem mirror, the potential structure of the whole plasma should be investigated. In particular, the potential structure from the plug-barrier cell to the end plate installed on the end wall of the vacuum vessel has physical import. The fundamental ECRH at the plug region generates an electromotive force by driving the axial flow of electrons. This electromotive force is divided into the positive plug potential and the negative end plate potential. This paper shows the variations of these potential with currents flowing through each region. The end plate potential increases with this current. Analysis of a current carrying sheath is applied to the end plate potential. The plug potential decreases with a radial ion current in the peripheral region of the plug-barrier cell. To illustrate this point, a plasma shot with NBI is examined in which a trapped plasma is generated in the cell.