A composite particle powder comprising an electron-conducting material and an ion-conducting material that are agglomerated together, the electron-conducting material being an electron-conducting oxide or a metal having catalytic activity and the ion-conducting material being an oxide having oxide ion-conductivity or an oxide having proton-conductivity; the composite particle powder having a volume-based 50% particle diameter of larger than 0.1 μm and 2.0 μm or less in particle size distribution measurement; the composite particle powder having a ratio of volume-based 75% particle diameter to 25% particle diameter, (D75/D25), of 1.1 to 2.2 in particle size distribution measurement; the composite particle powder having D50 and a BET specific surface area that satisfy the following: in the case of 0.1 μm < D50 of the composite particle powder ≤ 0.5 μm, the BET specific surface area of the composite particle powder is larger than 25 m 2 /g and 200 m 2 /g or less; in the case of 0.5 μm < D50 of the composite particle powder ≤ 0.9 μm, the BET specific surface area of the composite particle powder is larger than 5 m 2 /g and 50 m 2 /g or less; in the case of 0.9 μm < D50 of the composite particle powder ≤ 1.3 μm, the BET specific surface area of the composite particle powder is larger than 4 m 2 /g and 30 m 2 /g or less; and in the case of 1.3 μm < D50 of the composite particle powder ≤ 2.0 μm, the BET specific surface area of the composite particle powder is larger than 2 m 2 /g and 20 m 2 /g or less; and the composite particle powder having an ion-conducting material content of 35 to 75 mass%, wherein the particle size distribution of the composite particle powder is measured by laser diffraction particle size analysis and the volume-based D25, D50 and D75 in particle size distribution measurement are diameters of particles corresponding to the cumulative volumes at 25%, 50% and 75% from the lower end relative to the total volume, respectively.. the composite particle powder having a volume-based 50% particle diameter of larger than 0.1 μm and 2.0 μm or less in particle size distribution measurement; the composite particle powder having a ratio of volume-based 75% particle diameter to 25% particle diameter, (D75/D25), of 1.1 to 2.2 in particle size distribution measurement; the composite particle powder having D50 and a BET specific surface area that satisfy the following: in the case of 0.1 μm < D50 of the composite particle powder ≤ 0.5 μm, the BET specific surface area of the composite particle powder is larger than 25 m 2 /g and 200 m 2 /g or less; in the case of 0.5 μm < D50 of the composite particle powder ≤ 0.9 μm, the BET specific surface area of the composite particle powder is larger than 5 m 2 /g and 50 m 2 /g or less; in the case of 0.9 μm < D50 of the composite particle powder ≤ 1.3 μm, the BET specific surface area of the composite particle powder is larger than 4 m 2 /g and 30 m 2 /g or less; and in the case of 1.3 μm < D50 of the composite particle powder ≤ 2.0 μm, the BET specific surface area of the composite particle powder is larger than 2 m 2 /g and 20 m 2 /g or less; and in the case of 0.1 μm < D50 of the composite particle powder ≤ 0.5 μm, the BET specific surface area of the composite particle powder is larger than 25 m 2 /g and 200 m 2 /g or less; in the case of 0.5 μm < D50 of the composite particle powder ≤ 0.9 μm, the BET specific surface area of the composite particle powder is larger than 5 m 2 /g and 50 m 2 /g or less; in the case of 0.9 μm < D50 of the composite particle powder ≤ 1.3 μm, the BET specific surface area of the composite particle powder is larger than 4 m 2 /g and 30 m 2 /g or less; and in the case of 1.3 μm < D50 of the composite particle powder ≤ 2.0 μm, the BET specific surface area of the composite particle powder is larger than 2 m 2 /g and 20 m 2 /g or less; and the composite particle powder having an ion-conducting material content of 35 to 75 mass%, wherein the particle size distribution of the composite particle powder is measured by laser diffraction particle size analysis and the volume-based D25, D50 and D75 in particle size distribution measurement are diameters of particles corresponding to the cumulative volumes at 25%, 50% and 75% from the lower end relative to the total volume, respectively..