A waveguide/planar line transducer of the present invention includes a waveguide that transmits electromagnetic waves through an opening portion, and a multiplayer substrate that includes a plurality of conductive layers. The multilayer substrate includes: a first conductive layer that is in close contact with the opening portion of the waveguide, and includes a first coupling hole provided at a position overlapping the opening portion of the waveguide when viewed in a plate thickness direction of the multilayer substrate; a strip electrode that is electromagnetically coupled to the first conductive layer, arranged on an opposite side to the first conductive layer in the plate thickness direction, and extending in one of a planar direction of the multilayer substrate; and a second conductive layer that is arranged between the first conductive layer and the strip conductor in the plate thickness direction, and includes a second coupling hole having a protuberance facing at least one of directions in which the strip electrode extends.