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, wherein the first electrode mediator includes a tetrathiafulvalene derivative, and the tetrathiafulvalene derivative has a ring-forming substituent at positions 4,4′ and 5,5′ of a tetrathiafulvalene skeleton thereof.