A magnetoresistive effect oscillator is provided which can realize a rise or a fall of oscillation at a higher speed. In the magnetoresistive effect oscillator, at the rise, a current having a first current density, which is larger than a critical current density for oscillation, is applied, and thereafter a current having a second current density, which is less than the current density corresponding to the first current density and not less than the critical current density for oscillation, is applied such that the magnetoresistive effect element oscillates at a predetermined frequency. In the magnetoresistive effect oscillator, at the fall, starting from the state where a first current density is applied to hold the magnetoresistive effect element in an oscillating condition, a current having a second current density and having polarity reversed to that of the first current density is applied such that the oscillation disappears.