the present invention discloses a fuel - saving accelerator for internal combustion engine , which comprises a water tank for storing water therein as auxiliary fuel for the inner combustion engine ; a water processing unit communicating with the water tank to process the water supplied from the water tank ; and a water control unit which is connected between the water processing unit and a combustion chamber of the inner combustion engine to control the amount of the processed water supplied from the water processing unit and supply a controlled amount of water to the combustion chamber .