1. An apparatus for feedback controlling a controlled variable of a rotary machine based on a phase of an output voltage vector of a power converter including a switching element, the apparatus comprising: an interference-reduction current calculator configured to calculate, as an interference reduction current, a component of a current vector flowing in a coordinate axis in a rotating coordinate system defined with respect to a rotor of the rotary machine, the coordinate axis serving as an interference reduction coordinate axis, the component of the current vector flowing in the interference reduction coordinate axis and having reduced interference from change of the phase of the output voltage vector; an amplitude setter configured to set, as a manipulated variable for feedback controlling the interference reduction current to a command current value based on a command value for the controlled variable, one of: an amplitude of the output voltage vector, and an amplitude parameter indicative of a component of the output voltage vector, the component of the output voltage vector depending on the amplitude of the output voltage vector; and a switching unit configured to perform switching operations of the switching element of the power converter based on the manipulated variable and the phase of the output voltage vector, thus causing the controlled variable to follow the command value.