To provide a frequency synthesizer in which high responsiveness is maintained and which can be stably adapted to variation or diffusion in respective sections in spite of simplified hardware configuration.

A frequency synthesizer has a variable frequency oscillator which generates an output signal at a desired frequency corresponding to control information supplied by a phase-locked loop and includes a table in which an initial value of control information to be given to the variable frequency oscillator is registered beforehand for each target frequency that may become the desired frequency; initial setting means for regulating a response of the phase-locked loop when the desired frequency is switched, and supplying the initial value registered in the table correspondingly to a latest desired frequency to the variable frequency oscillator; lock-up detecting means for detecting lock-up of the phase-locked loop based on a phase difference; and phase lock resuming means for canceling the response regulation when the lock-up is detected, and regulating supply of the initial value by the initial value setting means.