A flow battery includes a first liquid containing a first electrode mediator dissolved therein, a first electrode immersed in the first liquid, a first active material immersed in the first liquid, and a first circulation mechanism that circulates the first liquid between the first electrode and the first active material. The first electrode mediator includes a carbazole derivative, and the carbazole derivative has a substituent at positions 3, 6, and 9 of a carbazole skeleton thereof.