A variable-frequency magnetoresistive effect element includes a magnetoresistive effect element, a magnetic-field applying mechanism that applies a magnetic field to the magnetoresistive effect element, an electric-field applying mechanism that applies an electric field to the magnetoresistive effect element, and a control terminal connected to the electric-field applying mechanism and used for applying a voltage that varies in at least one of magnitude and polarity to the electric-field applying mechanism. The magnetoresistive effect element contains an antiferromagnetic material or ferrimagnetic material having a magnetoelectric effect. A spin torque oscillation frequency or spin torque resonance frequency of the magnetoresistive effect element is controlled by varying the voltage applied via the control terminal in at least one of magnitude and polarity.