Abstract: A signal transmission device includes a first body made of electromagnetic shielding material and fixed to a rotary shaft which passes through the first body. The first body revolves together with the rotary shaft around an axis of the rotary shaft. A second body made of electromagnetic shielding material is stationarily disposed around the rotary shaft. A first conductive element is spirally formed on a first substrate in a manner that peak magnitude points of traveling wave component of transmission signal generated on the element are positioned out of alignment with each other between the neighboring turns of the spiral element in radial direction of the spirally formed element. The first conductive element is disposed in the first body. A second conductive element is formed on a second substrate and has a first width enough to cover the first conductive element in the radial direction and a second width in a direction perpendicular to the radial direction.