To provide a method of measuring the amount of an HF (Hydrogen Fluoride) in an electrolytic solution, by which method the amount of the HF in the electrolytic solution can be made constant; and to provide a method of manufacturing a lithium ion secondary battery using the method.

According to the method of measuring the amount of the HF in the nonaqueous electrolytic solution 140 of the lithium ion secondary battery 100, the nonaqueous electrolytic solution 140 is a nonaqueous electrolytic solution containing an ethylene carbonate. The method includes: a separating step (steps S45 and S47) of separating the components of the nonaqueous electrolytic solution 140 into the ethylene carbonate and other electrolytic components; and a neutralizing/titrating step (step S46) of neutralizing/titrating other electrolytic components under an inert gas atmosphere.

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