Abstract: To provide an electrolyte and a magnesium secondary battery that can embody a magnesium secondary battery having room-temperature operability and satisfactory cyclability. An electrolyte 13 including an organic solvent, magnesium salt and cyclic acid anhydride is provided. Cyclic acid anhydride is preferably contained at a concentration at least equimolar to magnesium salt, and is further preferably contained at a molar concentration 1.0 to 3.0 times that of magnesium salt. Further, a magnesium secondary battery 1 including these electrolyte 13 and a negative electrode 12 containing magnesium or magnesium alloy is provided.