An amorphous oxide-based positive electrode active material that is a production material of a positive electrode for an all-solid secondary battery, wherein the amorphous oxide-based positive electrode active material (i) comprises an alkali metal selected from Li and Na; a second metal selected from Co, Ni, Mn, Fe, Cr, V, Cu, Ti, Zn, Zr, Nb, Mo, Ru and Sn; an ionic species selected from phosphate ion, sulfate ion, borate ion, silicate ion, aluminate ion, germanate ion, nitrate ion, carbonate ion and halide ion; and an oxygen atom (except for the oxygen atom constituting the ionic species); (ii) contains at least an amorphous phase; and (iii) is a production material of a positive electrode with a thickness of 20 μm or more.