Transmission circuit, antenna duplexer, and radio-frequency switch circuit

A small-sized, high-performance transmission circuit is provided which avoids degradation in transmission line characteristics caused by coupling, due to a transmission line and a leader electrode to an external electrode which are oppositely facing, of the electromagnetic field induced by the transmission line and the electromagnetic field induced by the leader electrode. In order to attain the aforementioned object, the present invention comprises a first shield layer which is a first ground electrode, a second shield layer which is a second ground electrode, and a spiral-shaped transmission line facing the first shield layer and the second shield layer which is disposed between the first shield layer and the second shield layer. The spiral portion of the transmission line, when viewed from the top face or the bottom face of the transmission line, is disposed on the inside of the first shield layer and the second shield layer.