There is provided a digital controller which can reduce a variation in output voltage as compared to the conventional digital controller at the time of abrupt changes in load and in input of a power amplifier. The digital controller is built in the power amplifier for supplying an output voltage vo to a load and is equipped with a manipulated variable calculating unit which detects the output voltage vo to calculate a manipulated variable ξ1 and a signal generating unit which converts the manipulated variable ξ1 into a signal for making the power amplifier operate. A feedforward function from an equivalent disturbance qy caused by the variation in the load is replaced by a feedback function from the output voltage vo and the manipulated variable ξ1, thereby allowing a transfer characteristic from the disturbance qy to the output voltage vo to result in a quadratic differential characteristic. Consequently, the variation in output voltage can be reduced as compared to the conventional controller at the time of the abrupt changes in the load and in the input of the power amplifier.