TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Components: 1st electrolyte solution contains 0.4 mass% total aluminum, calcium, sodium, potassium, iron, silicon, and chromium, including tetravalent vanadium ion, and less than or equal to 0.1 ppm dissolved oxygen. Preferred Process: 1st electrolysis process uses constant current, where greater than or equal to 2 constant currents are applied or the current is increased by 1 in steps. 1st electrolysis process is performed by monitoring oxidation-reduction potential and switching from electrolysis with constant current to potentiostatic electrolysis when (A) electrolysis with constant current is performed with current density (absolute value) of 1-3 mA/cm2 in the initial stage applied to anode or cathode, and switching to potentiostatic electrolysis when oxidation-reduction potential of anode is +600 mV to +1100 mV, or when oxidation-reduction potential of cathode is +300 mV to -200 mV, or (B) electrolysis with constant current is performed with current density (absolute value) of 1-3 mA/cm2 in the initial stage applied to anode or cathode, adjusting the current density (absolute value) to 3-6 mA/cm2 and performing electrolysis with constant current when oxidation-reduction potential of anode is +600 mV to +850 mV, or when oxidation-reduction potential of cathode is +300 mV to plus minus 0 mV, and switching to potentiostatic electrolysis when oxidation-reduction potential of anode is +850 mV to +1100 mV, or when oxidation-reduction potential of cathode is plus minus 0mV to -200 mV. Potentiostatic electrolysis in 2nd electrolysis process is performed by applying greater than or equal to 2 constant voltages or increasing constant voltage by 1 in steps. 2nd electrolysis process is performed by monitoring oxidation-reduction potential and increasing voltage value in steps during (a) when switching from electrolysis with constant current to potentiostatic electrolysis, 1st potentiostatic electrolysis is performed by applying 1.4 V-1.5 V voltage between anode or cathode electrodes, and 2nd potentiostatic electrolysis is performed by applying 0.2 V-0.5 V voltage between the electrodes when current density falls to 0.2-1 mA/cm2 (absolute value) during 1st potentiostatic electrolysis, or (b) when switching from electrolysis with constant current to potentiostatic electrolysis, 1st potentiostatic electrolysis is performed by applying 1.4 V-1.5 V voltage between anode or cathode electrodes, performing 2nd potentiostatic electrolysis by increased the voltage between the electrodes to 0.2 V-0.5 V when current density falls to 0.2-1 mA/cm2 (absolute value) during 1st potentiostatic electrolysis, and performing 3rd potentiostatic electrolysis by increasing the voltage between the electrodes to 0.2 V-0.5 V when current density falls to 0.2-1 mA/cm2 (absolute value) during 2nd potentiostatic electrolysis. The process involves dissolving vanadium salt in 1st electrolyte solution to obtain vanadium ion solution and charging/discharging repeatedly with vanadium secondary battery. Electrolysis process involves bubbling 1st electrolyte solution which contains vanadium ion using inert gas, in vacuum. Vanadium ion solution for 1st electrolyte solution prepared by preparatory process to obtain vanadium electrolyte solution which contains pentavalent vanadium ion by oxidation electrolysis during electrolysis process, or to obtain vanadium electrolyte solution which contains bivalent vanadium ion by reduction electrolysis during electrolysis process.