An electrolysis method comprising: an electrolysis process of (S21A) supplying an electrolyte solution containing sulfuric acid ions to an anode chamber of a diaphragm electrolytic bath that includes an anode and a cathode disposed to face each other, and a diaphragm for partitioning the anode chamber in which the anode is positioned and a cathode chamber in which the cathode is positioned to thus generate peroxydisulfate through an anodic reaction, while a carbon dioxide gas or an electrolyte solution containing the gas dissolved therein is supplied (S21B) to the cathode chamber of the diaphragm electrolytic bath to thus generate formate; and a concentration-acidification process of supplying the formate generated in the cathode chamber of the diaphragm electrolytic bath , so that the formate is reacted with hydrogen ions supplied from an outside or supplied using self-generation to thus be converted into formic acid; wherein the concentration-acidification process includes: (S22) an acidification step of supplying the hydrogen ions necessary for the reaction using sulfuric acid from an external supply source so that the formate supplied from the diaphragm electrolytic bath is converted into the formic acid; and (S23) a separation step of separating and concentrating the formic acid and sulfate produced during the acidification step to thus produce the formic acid, and further comprising: (S24) supplying the sulfate separated during the separation step as a raw material of the anode chamber of the diaphragm electrolytic bath . an electrolysis process of (S21A) supplying an electrolyte solution containing sulfuric acid ions to an anode chamber of a diaphragm electrolytic bath that includes an anode and a cathode disposed to face each other, and a diaphragm for partitioning the anode chamber in which the anode is positioned and a cathode chamber in which the cathode is positioned to thus generate peroxydisulfate through an anodic reaction, while a carbon dioxide gas or an electrolyte solution containing the gas dissolved therein is supplied (S21B) to the cathode chamber of the diaphragm electrolytic bath to thus generate formate; and a concentration-acidification process of supplying the formate generated in the cathode chamber of the diaphragm electrolytic bath , so that the formate is reacted with hydrogen ions supplied from an outside or supplied using self-generation to thus be converted into formic acid; wherein the concentration-acidification process includes: (S22) an acidification step of supplying the hydrogen ions necessary for the reaction using sulfuric acid from an external supply source so that the formate supplied from the diaphragm electrolytic bath is converted into the formic acid; and (S23) a separation step of separating and concentrating the formic acid and sulfate produced during the acidification step to thus produce the formic acid, and further comprising: (S24) supplying the sulfate separated during the separation step as a raw material of the anode chamber of the diaphragm electrolytic bath . (S22) an acidification step of supplying the hydrogen ions necessary for the reaction using sulfuric acid from an external supply source so that the formate supplied from the diaphragm electrolytic bath is converted into the formic acid; and (S23) a separation step of separating and concentrating the formic acid and sulfate produced during the acidification step to thus produce the formic acid, and further comprising: (S24) supplying the sulfate separated during the separation step as a raw material of the anode chamber of the diaphragm electrolytic bath . <EOS>