A directional coupler includes a substrate, a main line 121, a main line 122, and a sub-line. The main line 121 and the main line 122 each include a conductor pattern formed in the substrate, and are connected in parallel to each other. The sub-line includes a conductor pattern formed in the substrate. The sub-line is disposed at a position at least partially overlapping with the main line 121 in a plan view of the substrate.