This paper presents an investigation of a magnetosphere-ionosphere (M-I) coupling process during magnetic storms, using observations of auroral kilometric radiation (AKR) in the field-aligned acceleration region and plasma density variation in the plasma sheet. The main phase electron precipitation, which AKR does not accompany, was confirmed as not characterized by inverted-V type precipitation but rather by precipitation with Maxwellian type distribution. This indicates the absence of the field-aligned potential in the M-I coupling region during the early phase of the magnetic storm. We found that the superdense plasma in the plasma sheet is closely concerned with the AKR activity; that is, the appearance of the superdense plasma sheet seems to suppress the AKR activity. On the basis of these observations we discuss the current-voltage relation between the magnetosphere and ionosphere. The occurrence of the superdense plasma sheet accompanied with the AKR disappearance was found to be caused by coronal mass ejection (CME) with magnetic cloud. The overall story from the arrival of CME at the Earth to the AKR disappearance in the M-I coupling region is presented.