A magnetoresistive effect oscillator executes a first step of applying a current, which has a first current density larger than a critical current density JO for oscillation, to a magnetoresistive effect element for a time TP, and then executes a second step of applying a current, which has a second current density JS smaller than the first current density and not smaller than the critical current density JO for oscillation, to the magnetoresistive effect element. The following formulae (1), (2) and (3), or the following formulae (1) and (4) are satisfied on an assumption that an average value of the first current density during the time TP in the first step is JP, a critical current density for magnetization reversal of the magnetoresistive effect element is JR, and a magnetization reversal time of the magnetoresistive effect element is TR: 0.1 × T R ⁡ ( J R - J O ) J p - J S