A method for producing a garnet type oxide solid electrolyte that is inhibited from a reaction of a flux and a crucible in heating and from a contamination with a crucible component produced by the reaction. The method for producing a garnet type oxide solid electrolyte represented by a general formula (Lia1, Aa2)La3-bEbZr2-cMcO12 may comprise the steps of: preparing raw materials for the garnet type oxide solid electrolyte at a stoichiometric ratio of the above general formula; preparing flux raw materials by using NaCl and KCl at a molar ratio of NaCl:KCl=x:(1−x) where x satisfies 0≤x≤1; mixing the solid electrolyte raw materials prepared in the above step and the flux raw materials prepared in the above step; and heating a mixture of the solid electrolyte raw materials and the flux raw materials at a temperature of less than 1100° C.