Significant effects of sheared transverse electric fields in plasmas on both turbulent fluctuations and drift waves are experimentally demonstrated with improvement in plasma confinement for the first time in the tandem mirror GAMMA 10. Here, electron-cyclotron heatings (ECH) for ion-confining potential formation are applied in association with a significant rise in the absolute value of the central-cell potential and the resulting formation of a strong shear of electric fields of the order of 10 kV/m2 in the radial direction of the plasma column (dEr/dr). The central-cell line density increases during ECH in association with decreasing fluctuations. Various fluctuation diagnostics, in particular, the frequency analyses of end-loss ion currents and central soft x-ray brightness, show the consistent features. This encourages the usefulness of potentials and radial electric-field shear for confinement improvements.