The present invention provides a frequency synthesizer capable of switching an oscillation frequency band while maintaining a lock state realized with a small-scale and low-current-consumption circuit configuration, and an oscillation control method of the frequency synthesizer. A frequency synthesizer having an oscillation control circuit capable of switching an oscillation frequency band, includes: a differential bias unit for generating, as a differential bias signal, a difference in a bias signal to the oscillation control circuit caused by variations in the oscillation frequency band; a band limit detector for detecting transition of the oscillation frequency, that exceeds a limit value of the oscillation frequency band; and a transient controller for performing control of gradually shifting an output bias signal on the differential bias unit in a transient period in which operation of the differential bias unit is switched for switching the oscillation frequency band in accordance with detection of the band limit detector.