To provide a coupling with a compact formation capable of blocking transmission of power from a driven member to an electric motor while ensuring transmission efficiency from the electric motor to the driven member.

A reverse preventive structure with low transmission efficiency such as a worm and a worm wheel is not required in the coupling 10. The coupling can block power transmission from a rotary shaft 3 to the electric motor 1 while power transmission from the electric motor 1 to the rotary shaft 3 is permitted. Consequently, capacity of the electric motor 1 can be reduced, and saving electricity is also achieved. Thus, the compact coupling 10 with a high function is provided by integrally assembling the reverse preventive structure to the coupling. The above-mentioned effects are achieved by simply interconnecting the rotary shaft 3 and an output shaft 1b of the electric motor 1 via the coupling 10. Assembling the coupling to the electric motor 1 is easy, and removal and installation of the coupling 10, which has a superior handling feature and is free from mounted places, is also easy after assembled with the electric motor 1.