The present invention relates to a method for producing an oxide containing a conductive mayenite type compound and having an electron concentration of 1×1018/cm3 or more, from a raw material which is a combination of a calcium compound and an aluminum compound or is a compound containing calcium and aluminum, each having a molar ratio of calcium oxide and aluminum oxide ranging from 9:10 to 14:5 in terms of the oxides, the method including the steps of: heating and holding the raw material at 900 to 1,300° C. to produce a calcined powder containing at least one oxide selected from the group consisting of a calcium aluminate, calcium oxide and aluminum oxide; and heating and holding the calcined powder at 1,200° C. to less than 1,415° C. under a reduction atmosphere in an inert gas atmosphere or a vacuum atmosphere each having an oxygen partial pressure of 1,000 Pa or less.