Composite metal oxide particles comprising: a first composite metal oxide represented by the following Chemical Formula 1; a second composite metal oxide represented by the following Chemical Formula 2; and a third composite metal oxide represented by the following Chemical Formula 3, wherein the content of the second composite metal oxide is 30 wt% to 70 wt% and the content of the third composite metal oxide is 30 wt% to 70 wt%, based on the total weight of the second and third composite metal oxides, wherein the composite metal oxide particles are particles in which a first peak having a 2θ value at 32° or more and 33° or less of an X-ray diffraction analysis graph, a second peak having a 2θ value at 31° or more and less than 32° of the graph, and a third peak having a 2θ value at 29.5° or more and 30.5° or less of the graph appear, wherein the first peak is a peak associated with the first composite metal oxide, wherein the second peak is a peak associated with the second and third composite metal oxides, wherein the third peak is a peak associated with the third composite metal oxide, and wherein a percentage of an intensity of the third peak is 30% or more and 70% or less based on an intensity of the second peak: [Chemical Formula 1] La 1-x Q x Ga 1-y Z y O 3-δ [Chemical Formula 2] LaQGaO 4 [Chemical Formula 3] LaQGa 3 O 7 in Chemical Formulae 1 to 3, Q is at least one of a monovalent metal having a radius which is 90% or more and 110% or less of a radius of a lanthanum atom, a divalent metal having a radius which is 90% or more and 110% or less of a radius of a lanthanum atom, and a tetravalent metal having a radius which is 90% or more and 110% or less of a radius of a lanthanum atom, Z is at least one of a monovalent metal having a radius which is 90% or more and 110% or less of a radius of a gallium atom, a divalent metal having a radius which is 90% or more and 110% or less of a radius of a gallium atom, and a tetravalent metal having a radius which is 90% or more and 110% or less of a radius of a gallium atom, and 0 < x < 0.25, 0 < y < 0.25, and 0 < δ < 0.5. a first composite metal oxide represented by the following Chemical Formula 1; a second composite metal oxide represented by the following Chemical Formula 2; and a third composite metal oxide represented by the following Chemical Formula 3, wherein the content of the second composite metal oxide is 30 wt% to 70 wt% and the content of the third composite metal oxide is 30 wt% to 70 wt%, based on the total weight of the second and third composite metal oxides, wherein the composite metal oxide particles are particles in which a first peak having a 2θ value at 32° or more and 33° or less of an X-ray diffraction analysis graph, a second peak having a 2θ value at 31° or more and less than 32° of the graph, and a third peak having a 2θ value at 29.5° or more and 30.5° or less of the graph appear, wherein the first peak is a peak associated with the first composite metal oxide, wherein the second peak is a peak associated with the second and third composite metal oxides, wherein the third peak is a peak associated with the third composite metal oxide, and wherein a percentage of an intensity of the third peak is 30% or more and 70% or less based on an intensity of the second peak: [Chemical Formula 1] La 1-x Q x Ga 1-y Z y O 3-δ [Chemical Formula 2] LaQGaO 4 [Chemical Formula 3] LaQGa 3 O 7 in Chemical Formulae 1 to 3, Q is at least one of a monovalent metal having a radius which is 90% or more and 110% or less of a radius of a lanthanum atom, a divalent metal having a radius which is 90% or more and 110% or less of a radius of a lanthanum atom, and a tetravalent metal having a radius which is 90% or more and 110% or less of a radius of a lanthanum atom, Z is at least one of a monovalent metal having a radius which is 90% or more and 110% or less of a radius of a gallium atom, a divalent metal having a radius which is 90% or more and 110% or less of a radius of a gallium atom, and a tetravalent metal having a radius which is 90% or more and 110% or less of a radius of a gallium atom, and 0 < x < 0.25, 0 < y < 0.25, and 0 < δ < 0.5. Q is at least one of a monovalent metal having a radius which is 90% or more and 110% or less of a radius of a lanthanum atom, a divalent metal having a radius which is 90% or more and 110% or less of a radius of a lanthanum atom, and a tetravalent metal having a radius which is 90% or more and 110% or less of a radius of a lanthanum atom, Z is at least one of a monovalent metal having a radius which is 90% or more and 110% or less of a radius of a gallium atom, a divalent metal having a radius which is 90% or more and 110% or less of a radius of a gallium atom, and a tetravalent metal having a radius which is 90% or more and 110% or less of a radius of a gallium atom, and 0 < x < 0.25, 0 < y < 0.25, and 0 < δ < 0.5.