To achieve favorable directivity in a directional coupler, even with a low magnetic-field coupling coefficient, the directional coupler includes a main line, a secondary line, and impedance conversion sections. The main line is connected between a signal input port and a signal output port. The secondary line is coupled to the main line through coupling capacitance and mutual inductance. The impedance conversion sections are connected between the secondary line and a coupling port or an isolation port, and the impedance viewed from the secondary line differs from the impedance viewed from a port side while both impedances viewed from the secondary line are equal.