An FLL circuit having a capability of configuring a desired loop bandwidth in a short period of time is provided. An FDC 17 generates a feedback of an output signal of a VCO 15. An error detector 11 detects an error of the output signal of the VCO 15. A voltage retainer 13 retains an output of a control voltage of the VCO 15. A reference signal generator 16 generates a reference signal. An adder 14 adds the reference signal to a control voltage outputted by the voltage retainer 13. A Kv calculator 18 calculates a gain Kv of the VCO 15 based on a degree of transition of an output frequency of the VCO 15. A loop bandwidth controller 19 adjusts, based on the gain Kv of the VCO 15, a gain of a loop filter 12 to an optimum value, and configures a desired loop bandwidth.