An output-voltage control device includes a comparator which generates a comparison result after a given time passes from first timing of a first periodic signal, the comparison result being obtained by comparing a difference between an output voltage and a reference voltage with the first periodic signal, a first signal generator which generates a timing control signal which is at a first level before the given time passes from the first timing and which changes from the first level to a second level in a period in which the comparator outputs the comparison result after the given time passes, and a second signal generator which generates a control signal for controlling the output voltage in accordance with the comparison result and the timing control signal.