To provide a frequency synthesizer for increasing variation resistance and compensating the frequency variation of a VCO when a temperature fluctuates.

The VCO 5, a phase comparator 2 for phase-comparing signals 22 for which the oscillation output of the VCO is frequency-divided in a variable frequency divider circuit 4 with the reference signals 21 of a reference oscillation circuit 1, and a loop filter 3 for generating a control voltage 23 to the VCO on the basis of a phase-comparison result configure a PLL, and the VCO includes a capacity bank having a plurality of weighted capacitive elements 56-58 and a plurality of switches to be turned on/off on the basis of control signals, and a varactor 54. A temperature compensation block 7 includes a correction potential generation circuit 73 for the varactor, a correction potential generation circuit 71 for the parasitic capacitance of the capacity bank, a gain variable amplifier for weighting the output potential of the correction potential generation circuit on the basis of the control signals, and an addition circuit 79 for adding the output voltage of the correction potential generation circuit for the varactor and the output voltage of the gain variable amplifier. The varactor of the VCO is controlled by the output 25 of the addition circuit.