A 90-degree hybrid circuit includes a dielectric substrate, a first conductor that includes a conductor pattern formed on the dielectric substrate and electrically conducts between a first port and a second port, a second conductor that includes a conductor pattern formed on the dielectric substrate and electrically conducts between a third port and a fourth port, and a coupled line comprising a portion of the first conductor and a portion of the second conductor that face each other on front and back sides of the dielectric substrate. A first coupled line portion as the portion of the first conductor includes a coplanar line with first ground patterns formed to sandwich the first coupled line portion from both sides. A second coupled line portion as the portion of the second conductor includes a coplanar line with second ground patterns formed to sandwich the second coupled line portion from both sides.