A generation controller controls an amount of power generated by an on-vehicle generator responsively to a command signal from an external controller. The generation controller comprises detection, determination, and output control units. The detection unit detects a revolution number of the generator. The determination unit determines whether or not the revolution number of the generator is over a reference revolution number. The output control it controls a generator output based on a first target value decided corresponding to a command value indicated by the command signal, when the revolution number of the generator is less than the reference revolution number, and controls the generator output based on a second target value in place of the first target value. The second target value is set internally within the generation controller, when the revolution number of the generator is over or equal to the reference revolution number.