A damping device for electric railway poles has been invented in order to reduce vibration in overhead contact lines caused by earthquakes or passing trains. The authors conducted theoretical analyses on poles equipped with the damping device and vibration tests on full scale poles. The damping device is a sandwich structure consisting of several steel plates and viscoelastic sheets and has a damping effect even for minute displacement. It has been confirmed that the theoretical results are almost identical to results of vibration tests on actual steel-pipe pole. Vibration tests also demonstrated that the damping device increases the loss factor ten-fold in comparison to a simple steel-pipe pole. In vibration tests on a concrete pole, the damping device reduces the maximum bending moment by approximately one half in comparison to a simple concrete pole.