A resonant coupler includes, on the main surface of a first dielectric substrate: a first resonant line disposed in a circumferential shape and having proximate first and second ends; an input line into which a signal is inputted; and a first connecting line that grounds a first end of the first resonant line, and, similar to the first dielectric substrate, includes, on the main surface of a second dielectric substrate: a second resonant line; an output line; and a second connecting line. When viewed in a direction perpendicular to the main surface of the first dielectric substrate, the first resonant line and the second resonant line have substantially matching contours, and the first resonant line and the second resonant line have a combined shape that is symmetrical about a line.