A directional coupler includes: a main strip line connected between a first input terminal and a first output terminal and transmitting high-frequency signals; a sub strip line connected between a second input terminal and a second output terminal, located parallel to the main strip line, and electromagnetically coupled to the main strip line; and a first capacitor connected in parallel with the main strip line or the sub strip line, wherein an LC resonant circuit is constituted by inductances of the main strip line and sub strip line and capacitance of the first capacitor, and the LC resonant circuit resonates with respect to a high-frequency signal propagating from the first input terminal to the second output terminal.