To easily add a transmission line that has a band inhibition characteristic.

The transmission line includes a line conductor formed on a dielectric substrate, a pair of ground conductors which are formed on the same surface as the line conductor of the dielectric substrate and are provided on both sides of the line conductor with a predetermined interval, a flat conductor plate which faces the line conductor and the ground conductor, and is provided to cover at least a portion, and a connection conductor which electrically connects the flat conductor plate and the pair of ground conductors by at least two points. The connection conductor is provided in a wiring direction of the line conductor with a predetermined interval.