Provided is an electric transmission device of a relative rotating unit that does not cause disconnection, operating temperature, and environmental problems, can prevent magnetic flux leakage, and can efficiently transmit power and signals without increasing the size. An electric transmission device of a relative rotation section includes a pair of magnetic elements (1A, 1B) rotatable relative to each other on the same axis (O). Each of the pair of magnetic elements 1A and 1B includes coils 3A and 3B and cores 2A and 2B, respectively. The coils 3A and 3B are magnetically coupled to each other, and transmit one or both of power and an electric signal. The cores 2A, 2B of the pair of magnetic elements 1A, 1B have cylindrical portions 2Aa, 2Ba each partially or wholly in the axial direction overlapping each other outside the gap G of the magnetic path. [Selection diagram] Fig. 1【課題】断線や使用温度、環境上の問題が生じず、かつ磁束漏れが防止できて、大型化することなく効率良く電力や信号の伝送が行える相対回転部の電気伝送装置を提供する。【解決手段】この相対回転部の電気伝送装置は、互いに同一の軸心Ｏ上で相対回転可能な一対の磁性素子１Ａ，１Ｂを備える。これら一対の磁性素子１Ａ，１Ｂは、それぞれコイル３Ａ，３Ｂとコア２Ａ，２Ｂとでなり、コイル３Ａ，３Ｂが互いに磁気結合され、電力および電気信号のいずれか一方または両方を伝送する。一対の磁性素子１Ａ，１Ｂのコア２Ａ，２Ｂは、磁路のギャップＧよりも外周に、軸方向の一部または全部が互いに２重となる円筒部２Ａａ，２Ｂａをそれぞれ有する。【選択図】図１
