The present invention relates to a power generation controller of a vehicle power generator, which includes: voltage detection means for sampling a voltage of a battery based on a reference clock signal; duty range setting means for setting a range for an ON/OFF ratio of a field switching element from the rotation speed of the power generator; control duty computation means for computing the ON/OFF ratio of the field switching element based on thus set duty range from a deviation between a power generation target voltage and voltage information as a result of sampling; and field PWM drive means for controlling the ON/OFF switching operation of the field switching element with an interval based on the reference clock signal from the computed ON/OFF ratio.