In an apparatus for controlling a variable of a rotary machine based on an AC voltage supplied to the rotary machine via a switching element of a power converter, a generator generates a drive signal including an on-off pattern of the switching element. A driver drives, based on the on-off pattern of the drive signal, the switching element. A parameter monitor monitors a parameter indicative of change of a harmonic current flowing in the rotary machine based on a harmonic voltage included in the AC voltage. A limiter limits, based on the parameter monitored by the parameter detector, generation of the drive signal by the generator to limit an increase of a level of the harmonic current.