The present invention provides a method for producing an electrolyte which is capable of retaining a high magnesium ion concentration. A method for producing an electrolyte in accordance with an aspect of the present invention comprises the step of: mixing a solvent, metal magnesium, and an elemental halogen, the metal magnesium being a metal containing magnesium in an amount of not less than 96% by weight with respect to 100% by weight of a total weight of the metal.