NOVELTY - An oxygen-reduction catalyst comprises composite particles having a portion containing inorganic metallic compound and a portion containing carbon. The composite particles contain metallic element (M1), carbon, and oxygen as constituent elements. The element (M1) is chosen from group II-VII elements and group XI-XV elements. The content of carbon atoms in the composite particles is 1-10 moles. The ratio of minimum value of spectral intensity in the region (V) between G band and D band with respect to peak intensity of G band in Raman spectrum is 0.1-0.35.    USE - Oxygen-reduction catalyst is used in reaction for converting oxygen into water, and preparation of ink for forming catalyst layer of electrode used in membrane electrode assembly of fuel cell (all claimed) e.g. polymer electrolyte fuel cell. Uses include but are not limited to motor vehicle, street light, mobile telephone, furniture, outdoor generator, cooler box, cardiac pacemaker, bulletin board, interior lighting of building, fluorescent lamp, etc.    ADVANTAGE - The oxygen-reduction catalyst improves the start-stop durability of the fuel cells.    DETAILED DESCRIPTION - An oxygen-reduction catalyst comprises composite particles having a portion containing inorganic metallic compound and a portion containing carbon. The composite particles contain metallic element (M1), carbon, and oxygen as constituent elements. The element (M1) is chosen from group II-VII elements and group XI-XV elements. The content of carbon atoms and oxygen atoms in the composite particles is 1-10 moles and 1-3 moles, respectively, with respect to total content of atoms contained in the element (M1). The ratio of minimum value of spectral intensity in the region (V) between G band and D band with respect to peak intensity of G band in Raman spectrum is 0.1-0.35. INDEPENDENT CLAIMS are included for the following:    (1) ink, which is obtained by dispersing the oxygen-reduction catalyst in a solvent; and    (2) membrane electrode assembly, which consists of a cathode catalyst layer, an anode catalyst layer, and a polymer electrolyte membrane provided between the anode and cathode catalyst layers formed using the ink.