In the tandem mirror GAMMA10, confining potential is formed at the plug region in order to decrease the loss region which exists in the velocity space of ion. Furthermore to increase the confining potential effectively, the electron which flows into the plug cell from the central cell is decreased by forming a potential dip (thermal barrier potential) between the central cell and the plug cell. The electrostatic potential at the inner mirror throat (IMT) of the plug/barrier cell may decrease and act as effective thermal barrier potential because of the effects of the strongest magnetic field and the anisotropy of ion temperature in the central cell. Simultaneous measurements of both the potential and the density in the IMT region are important to investigate the potential formation mechanism.