A flow battery includes: a first liquid containing a first nonaqueous solvent; a first electrode that is at least partly immersed in the first liquid; a second electrode which is a counter electrode to the first electrode; and a separator isolating the first electrode from the second electrode. The separator contains a lithium ion conductor. The lithium ion conductor contains a compound including main chains. At least one main chain of the main chains includes one or more aromatic rings and is cross-linked to at least another main chain of the main chains. At least one aromatic ring of the one or more aromatic rings includes one or more sulfo groups.