To provide a method for preparing a mayenite-containing oxide containing a mayenite type compound and having a hydride ion density of at least 1×1018/cm3 without need for expensive facilities, control of complicated reaction conditions or a long period of reaction time. A method for preparing a mayenite-containing oxide, which comprises a firing step of heating a starting material having a molar ratio of CaO:Al2O3 being from 9:10 to 14:5 based on the oxides at a temperature of from 900 to 1,300° C. to obtain a fired powder and a hydrogenation step of firing the fired powder at a temperature of at least 1,210° C. and lower than 1,350° C. in a hydrogen-containing gas having an oxygen partial pressure of at most 1,000 Pa to obtain a mayenite-containing oxide containing a mayenite type compound and having a hydride ion density of at least 1×1018/cm3, and a method for preparing an electroconductive mayenite-containing oxide, which comprises irradiating the obtained mayenite-containing oxide with an ultraviolet ray etc. to obtain a mayenite-containing oxide containing an electroconductive mayenite type compound.