Abstract: A multiband PLL frequency synthesizer is disclosed which includes: a reference signal generation circuit for generating a reference signal of which frequency is controlled; a phase comparator for generating a phase difference signal; a low-pass filter circuit for low-pass-filtering the phase difference signal with one of a plurality of cutoff frequencies selected; a VCO for generating and outputting a LO signal according to an output of the low-pass filter; a frequency dividing circuit having an integer frequency dividing mode and a fraction frequency dividing mode to supply the frequency-divided signal to the phase comparator; and a control circuit for supplying the reference frequency control signal to the reference signal generation circuit, a filter control signal to the low-pass filter circuit, and frequency dividing control signal (data) to the frequency dividing circuit in accordance with a frequency command signal.