The vehicle power-generation control unit includes a voltage detecting circuit detecting a generation voltage of a vehicle generator driven by a vehicle engine, a switching device allowing an exciting current to flow into an exciting winding of the vehicle generator when the switching device is in an on state, an average duty ratio detecting circuit detecting a temporal average duty ratio of the switching device on the basis of the generation voltage, an enhanced duty-cycle signal generating circuit generating an enhanced duty-cycle signal having a duty ratio equal to the temporal average duty ratio added with a supplementary duty ratio determined depending on at least one of vehicle state information and vehicle generator state information, and a driver circuit driving the switching device with the duty ratio of the enhanced duty-cycle signal when the generation voltage detected by the voltage detecting circuit is lower than a predetermined target voltage.