Mirror devices, having open-ended regions, provide intrinsic important advantages in terms of the control of radial-potential or E_r shear profiles on the basis of axial particle loss control for ambipolar potential formation. From the viewpoint of the mechanism investigation of radially localized transport-barrier formation, an idea of off-axis-resonant electron-cyclotron heating (ECH) is proposed and applied in the tandem mirror GAMMA 10, by the use of the aforementioned intrinsic advantage of mirror devices. The off-axis ECH produces a cylindrical layer with energetic electrons and facilitates the formation for a localized bumped ambipolar potential with a strong shear of radial electric fields E_r. Remarkable effects of dE_r/dr on the suppression of turbulent fluctuations with plasma confinement improvement are found by the use of signals of end-loss currents flowing from the central cell as well as the central-cell soft x ray. In association with the reduction of the fluctuations due to strong E_r shear formation, increases in ion and electron temperatures are found.