The formation of a plasma potential is experimentally investigated in a fully-ionized collisionless plasma flow along converging magnetic-field lines in the presence of a single ECR point. When the ECR occurs in the region of converging region, the potential profile is observed to be drastically modified. The resultant potential structure consists of a negative potential dip and a subsequent positive potential hump working as a plasma-flow dike potential, which persists in the steady state when the ECR point is located in a region of good curvature of the magnetic configuration. However, this potential structure temporally collapses when the ECR point is located in a bad curvature region. The phenomenon is considered to be caused by low-frequency flute and drift instabilities.