In an ADPLL frequency synthesizer where a frequency control word is changed from FCW0 to FCW2, a control sensitivity estimation section firstly measures oscillatory frequencies f1L and f1H obtained, respectively, when frequency control words FCW1L and FCW1H being used as dummies are set, and then measures an oscillatory frequency f2 obtained when a frequency control word FCW2 is set. Thereafter, based on values of the oscillatory frequencies f1L, f1H and f2, the control sensitivity estimation section calculates a control sensitivity KDCO2 obtained when the frequency control word FCW2 is set. Based on a value of the control sensitivity KDCO2, the loop filter determines values of filter coefficients α2 and ρ2 so as to be equal to a natural frequency ωn and a damping factor ζ, respectively, both of which have been previously designed.