A radio transmission system comprising: a first electronic device; and a second electronic device having a mounting structure mounted with said first electronic device, wherein a millimeter wave signal transmission line capable of transmitting information in a millimeter wave band is formed between said first electronic device and said second electronic device when said first electronic device is mounted in said mounting structure of said second electronic device, between said first electronic device and said second electronic device, a transmission object signal is converted into a millimeter wave signal and then the millimeter wave signal is transmitted via said millimeter wave signal transmission line, said mounting structure has a position defining section configured to define a mounted state of said first electronic device by a fitting structure, said first electronic device has a position defining section corresponding to said position defining section on a side of said mounting structure, said millimeter wave signal transmission line is formed in said position defining section (198A, 298A) of one of said mounting structure and said first electronic device, said millimeter wave signal transmission line has a structure that transmits the millimeter wave signal while confining the millimeter wave signal in the transmission line, said millimeter wave signal transmission line is a dielectric transmission line formed by a dielectric material having a characteristic of being capable of transmitting the millimeter wave signal, a shielding material for suppressing external radiation of the millimeter wave signal is disposed on a periphery of said dielectric material, and the dielectric material has a relative dielectric constant of 2 to 10 and a dielectric loss tangent of 0.00001 to 0.01. a first electronic device; and a second electronic device having a mounting structure mounted with said first electronic device, wherein a millimeter wave signal transmission line capable of transmitting information in a millimeter wave band is formed between said first electronic device and said second electronic device when said first electronic device is mounted in said mounting structure of said second electronic device, between said first electronic device and said second electronic device, a transmission object signal is converted into a millimeter wave signal and then the millimeter wave signal is transmitted via said millimeter wave signal transmission line, said mounting structure has a position defining section configured to define a mounted state of said first electronic device by a fitting structure, said first electronic device has a position defining section corresponding to said position defining section on a side of said mounting structure, said millimeter wave signal transmission line is formed in said position defining section (198A, 298A) of one of said mounting structure and said first electronic device, said millimeter wave signal transmission line has a structure that transmits the millimeter wave signal while confining the millimeter wave signal in the transmission line, said millimeter wave signal transmission line is a dielectric transmission line formed by a dielectric material having a characteristic of being capable of transmitting the millimeter wave signal, a shielding material for suppressing external radiation of the millimeter wave signal is disposed on a periphery of said dielectric material, and the dielectric material has a relative dielectric constant of 2 to 10 and a dielectric loss tangent of 0.00001 to 0.01.