A synthesis gas production system for producing CO and H2 by electrolyzing an aqueous solution containing CO2 includes: an electrolysis device including an anode chamber and a cathode chamber separated by a separator membrane; a cathode-side circulation line connected to the cathode chamber to circulate a cathode solution containing CO2; a catalyst supply device provided in the cathode-side circulation line to supply a CO production catalyst to the cathode solution; and a gas composition detection device configured to measure a ratio between CO and H2 in a production gas produced in the cathode chamber. At least one of control of a supply amount of the CO production catalyst by the catalyst supply device and control of a voltage applied between the anode and the cathode by the electrolysis device is performed to make a ratio of H2 to CO in the production gas be within a predetermined target range.