NOVELTY - A redox flow cell consists of charging/discharging cell (11), tank (23) which stores positive electrode electrolyte solution (22), tank (33) which stores negative electrode electrolyte solution (32), supply pipe (24) and supply pipe (34). The positive electrode electrolyte solution comprises redox system material of iron and acid. The negative electrode electrolyte solution comprises redox system material of titanium and acid, or redox system material of copper and amine. Acid in the positive and negative electrode electrolyte solution is chosen from citric acid and lactic acid.    USE - Redox flow cell (claimed).    ADVANTAGE - The redox flow cell has excellent charging/discharging cycle characteristics, coulomb efficiency, voltage efficiency and energy efficiency, and is economical.    DETAILED DESCRIPTION - A redox flow cell consists of charging/discharging cell, tank (23) which stores positive electrode electrolyte solution, tank (33) which stores negative electrode electrolyte solution, supply pipe (24) which supplies the positive electrode electrolyte solution to the charging/discharging cell, and supply pipe (34) which supplies the negative electrode electrolyte solution to the charging/discharging cell. The positive electrode electrolyte solution comprises redox system material of iron and acid. The negative electrode electrolyte solution comprises redox-system material of titanium and acid, or redox system material of copper and amine. The acid in the positive electrode electrolyte solution and the negative electrode electrolyte solution is chosen from citric acid and lactic acid. The dissolved oxygen amount in the negative electrode electrolyte solution in tank (23) is 1.5 mg/L or less. The amine is bis(2-aminoethyl)amine derivative of formula (I).    n=0-4;and    R1-R4=H, methyl or ethyl.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of redox flow cell.    Charging/discharging cell (11)    Positive electrode electrolyte solution (22)    Tanks (23,33)    Supply pipes (24,34)    Negative electrode electrolyte solution (32)    Case (41)