A directional coupler 1 includes: a first waveguide 31 and a second waveguide 32 that are arranged in parallel with post arrays 23 and 24 therebetween; a coupling window 33 that is formed in the post arrays 23 and 24 and electromagnetically couples the first waveguide 31 and the second waveguide 32; and a projecting conductor 50 that projects, at a center in a width direction of the coupling window 33, from one conductive layer 13 of paired conductive layers 12 and 13 facing each other in the first waveguide 31 and the second waveguide 32, toward an inside of the coupling window 33, the projecting conductor 50 being spaced from the other conductive layer 12.