There is provided a metal oxygen battery which uses an oxygen-storing material containing YMnO3 as a positive electrode material, and can reduce the discharge overpotential. The metal oxygen battery 1 has a positive electrode 2 to which oxygen is applied as an active substance, a negative electrode 3 to which metallic lithium is applied as an active substance, and an electrolyte layer 4 interposed between the positive electrode 2 and the negative electrode 3. The positive electrode 2 contains, as an oxygen-storing material, a composite metal oxide obtained by crushing and mixing a yttrium salt, a manganese salt and an organic acid, primarily calcining the mixture, and thereafter, adding a zirconium salt to the obtained primarily calcined material, and secondarily calcining the mixture, the composite metal oxide containing YMnO3 and ZrO2.