A plasma potential formation is investigated in a fully ionized collisionless plasma flow in the presence of electron cyclotron resonance (ECR) heating (ECRH) under simple diverging magnetic-field configurations. When ECR takes place in a diverging region, there appears a strong potential drop along the field lines, which results from a field-aligned electron acceleration, being accompanied by an effective ion acceleration so as to maintain the charge neutrality condition in the downstream region. Time-resolved measurements are performed in order to clarify dynamics of the potential formation.