A transmission line in which a waveguide tube and a planar transmission path are coupled to a post-wall waveguide broadens a band in which return loss is small. A transmission line (1) includes: a PPW (filter 11) including wide walls (13, 14) and narrow walls (16); and a waveguide tube (21). The PPW (filter 11) includes a columnar conductor (pin 18) that passes through an opening (13a) which is provided in the wide wall (conductor layer 13) and that has one end portion (181) located inside the substrate (12). The waveguide tube (21) is placed such that the columnar conductor (pin 18) passes through an opening (22a) and such that another end portion (182) of the columnar conductor (pin 18) is located inside the waveguide tube (21).