An apparatus for controlling an on-vehicle generator is provided. The apparatus comprises a switching element, current detecting unit, setting unit and driving unit. The switching unit turns on/off an exciting current to an exciting winding of the generator, the switching element being driven responsively to a driving signal having an adjustable duty ratio. The setting unit sets the duty ratio based on (i) a difference between an output voltage of the generator and a target voltage value and (ii) a condition in which an electric load is disconnected from the generator, wherein the duty ratio is allowed to lower than a minimum value of the duty ratio depending on a generated state of the generator. The minimum value allows the exciting current to flow at a lower limit detectable by the current detecting unit, The driving unit drives the switching element based on the driving signal.