Provided is a catalyst that is not corroded in an acidic electrolyte and/or under high electric potential, has excellent durability, and has high oxygen-reducing capability. The catalyst is formed from a niobium-containing oxycarbonitride, wherein I2/(I1+I2) is 0.25 or greater when I1 serves as the maximum X-ray diffraction intensity at a diffraction angle 2theta between 25.45 DEG and 25.65 DEG and I2 serves as the maximum X-ray diffraction intensity at a diffraction angle 2theta between 25.65 DEG and 26.0 DEG as measured by powder X-ray diffraction (Cu-Ka rays).