There is provided a method for producing synthesis gas without CO2 emissions. In the method for producing synthesis gas by reforming a hydrocarbon gas, a reforming reaction is caused by supplying a light hydrocarbon gas containing steam and/or carbon dioxide added thereto to a catalyst-filled tube side of a shell-and-tube heat exchanger-type reformer and circulating a heating medium, such as a molten salt, heated by a heat source such as solar heat or nuclear heat in a shell side of the shell-and-tube heat exchanger-type reformer. Carbon dioxide is removed from a produced gas discharged from the tube side and is supplied to the upstream side of the tube side and recycled.