A short slot directional coupler includes a dielectric substrate having both surfaces each covered by a metal film, a first via-hole string and a second via-hole string, each of which has via-holes penetrating through the substrate, and formed so that a distance between the first via-hole string and the second via-hole string is narrow at a center of a length direction of the string and wider along directions of both ends of the string, and a pair of third via-hole strings each having via-holes penetrating through the substrate, and formed between parts adjacent to both ends of the first via-hole string and parts adjacent to both ends of the second via-hole string to form a first post-wall waveguide along with the first via-hole string and a second post-wall waveguide along with the second via-hole string.