A directional coupler (10) includes a main line (20), a sub-line (40), and a variable capacitor (60). At least part of the sub-line (40) is disposed along the main line (20). The variable capacitor (60) is connected between the main line (20) and the sub-line (40). The directional coupler (10) achieves a stable degree of coupling between the main line (20) and the sub-line (40).