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During excitation of an AC superconducting coil, cyclical electromagnetic force is applied to the AC winding. The force causes mechanical vibration of the winding, and hence mechanical loss due to the vibration occurs. We have measured the mechanical loss under the conditions that were two kinds of bobbin materials whose surfaces were low and high frictional coefficients. The experimental results showed that frictional heating on the surface of the low-frictional-coefficient material increased with the gain of the contact force between the wire and the material. On the other hand, the heating on the high-coefficient material was decreased when the contact force increased.