A frequency synthesizer includes a digitally-controlled oscillator and an oscillation frequency control unit. The digitally-controlled oscillator includes a loop-shaped transmission line path having an odd number of parallel portions in each of which two conductors are arranged in parallel to each other with a space therebetween, and an odd number of intersection portions in each of which two conductors intersect spatially, an active circuit coupled between the two conductors, and a first variable capacitance unit and a second variable capacitance unit. The oscillation frequency control unit includes a ΣΔ modulation circuit for subjecting to ΣΔ modulation a first control signal for switching a high capacitance state and a low capacitance state of a first variable capacitance element included in the first variable capacitance unit.