To provide an electromagnetic flow coupler adapted to a practical use, and its designing method.

The electromagnetic flow coupler is constituted such that: a generator-side channel 2 where a liquid metal flows in a reverse direction and a pump-side channel 3 are arranged in parallel with each other; magnetic flux that crosses the generator-side channel 2 and the pump-side channel 3 is generated; an induction current is generated by the flow of the liquid metal in the generator-side channel 2; and the driving force of the liquid metal in the pump-side channel 3 is generated. When a flow rate in the generator-side channel 2 is not larger than 100 m3/min, a magnetic Reynolds number when the ratio of the flow rates of the generator-side channel 2 and the pump-side channel 3 is 0 is not larger than 0.5, and non-magnetic gap width Dm/channel width Dh is not more than 10. When the flow rate in the generator-side channel 2 exceeds 100 m3/min, non-magnetic gap width Dm/channel width Dh is 10.