A dielectric line includes a line portion and a surrounding dielectric portion. The line portion is formed of a first dielectric having a first relative permittivity. The surrounding dielectric portion is formed of a second dielectric having a second relative permittivity. The line portion propagates one or more electromagnetic waves of one or more frequencies within the range of 1 to 10 GHz. In a cross section orthogonal to the direction of propagation of the one or more electromagnetic waves through the line portion, the surrounding dielectric portion is present around the line portion. The first relative permittivity is 1,000 or higher. The second relative permittivity is lower than the first relative permittivity.