A secondary battery includes a first electrode; a first active material fluid which is electrically connected to the first electrode, contains a first active material and a supporting salt, and is flowable; and a second electrode including a structure which is formed by containing a second active material, the structure either being immersed in the first active material fluid or holding the first active material fluid, and a separating membrane disposed between the first active material fluid and the structure, the separating membrane having ion conducting properties and insulating properties.