In an oscillation control apparatus for an ultrasonic oscillator, an FET circuit is connected to an oscillation circuit for varying a resistant value of a resistor unit of the oscillation circuit, which determines a free-running frequency thereof. A phase-difference detecting circuit is provided for detecting a relative phase difference between voltage and current in the ultrasonic oscillator. A threshold discrimination circuit outputs a signal when the phase difference inputted from the phase-difference detecting circuit exceeds at least a predetermined value. A pump circuit integrates the signal from the threshold discrimination circuit. A resistant value of the FET circuit varies when an output signal from the pump circuit exceeds a predetermined value.