A directional coupler used in a predetermined frequency band includes a main line connected between a first terminal and a second terminal. A first sub line is connected to a third terminal and is electromagnetically coupled to the main line. A second sub line is connected to a fourth terminal and is electromagnetically coupled to the main line. A low pass filter is connected between the first sub line and the second sub line and causes a phase shift to be generated in a passing signal passing therethrough in such a manner that the phase shift monotonically increases within a range from about 0 to about 180 degrees with increasing frequency in the predetermined frequency band.