In an electric power supply control apparatus for a motor in a hybrid vehicle or an electric vehicle, when a main relay is switched over from an on-state to an off-state, only one of its positive-side switch and negative-side switch is turned off to provide a one-side off-state. Thus the numbers of operations of its switches are reduced in comparison to a system, in which both switches are turned off. When a charging relay is switched over from an on-state to an off-state, only one of a positive-side switch and a negative-side switch is turned off to provide a one-side off-state. Thus the numbers of operations of its switches are also reduced.