Based on the high-resolution spectroscopic measurements of the radial electric field (E r) in the JT-60U tokamak, we have identified the mechanisms centered around the role of the E r-curvature on the edge transport barrier (ETB) formation. In addition, we recognized a new role of the E r-shear for the expansion of pedestal width, compensating an unfavorable effect of the E r-curvature having its sign dependence on the turbulence transport. We found a new interesting phenomenon of very slow out-of-phase oscillations between the E r and ion temperature gradient in the frequency range of 30–50 Hz, preceding the slow L-H transition, which can demonstrate a causality test of the ETB formation in association with the non-uniformity E r-effects. A critical condition for the E r-bifurcations is also discussed.