PURPOSE: To obtain an improving agent for fuel able to reduce fuel consumption and increase output by making sea water to acidic, adding strong alkaline agent, removing generated precipitation, further removing water and dissolving the resultant solid in solvent.

CONSTITUTION: Sea water is adjusted to low pH by strong acid containing sulfate ion and strong alkaline agent such as sodium hydroxide is added to the sea water to be high pH, then precipitation generated in said process is separated from the solution. The solution after removing the precipitation is boiled to be 10-15% of original and generated precipitation is further removed, then water is completely removed from residual solution to afford the aimed solid improving agent.