A lithium isotope separation device includes a mixing container 10 for bringing a medium 1 and a medium 2 into contact with each other to mix them, a mixing means 11 configured to bring the medium 1 and the medium 2 into contact with each other by allowing the medium 1 and the medium 2 to flow in the mixing container 10 to mix them, and extraction means 10a and 10b configured to separate the mixed respective liquid media by utilizing a difference in specific gravity of respective liquid media and extract each of the separated and concentrated lithium isotopes.