An electrode catalyst having a core-shell structure comprising: a support; a core part formed on said support; and a shell part formed to cover at least a part of a surface of said core part, characterized in that when said shell part employs a one-layered structure, said support contains an electrically conductive carbon, said shell part contains platinum (Pt) and said core part contains palladium (Pd), and when said shell part employs a two-layered structure, said shell part has: a first shell part containing palladium (Pd) formed to cover at least a part of a surface of said core part; and a second shell part containing platinum (Pt) formed to cover at least a part of a surface of said first shell part, and that said electrode catalyst concurrently fulfils conditions expressed by the following formulae and: X 1 / M ≤ 1.2 X 2 / M ≤ 47.0 and that (X2/M) in the formula exceeds 4.5, wherein in the formula and the formula, M represents an amount of substance (number of atoms) of one or more constituent metal elements of the shell part or the second shell part, as calculated using an electrochemical surface area (ECSA) based on a hydrogen desorption wave obtained by cyclic voltammetry, X1 represents an amount of substance (number of atoms) of bromine (Br), as calculated based on the content of a bromine (Br) species as measured by X-ray fluorescence spectrometry, and X2 represents an amount of substance (number of atoms) of chlorine (Cl), as calculated based on the content of a chlorine species, as measured by X-ray fluorescence spectrometry. a support; a core part formed on said support; and a shell part formed to cover at least a part of a surface of said core part, characterized in that when said shell part employs a one-layered structure, said support contains an electrically conductive carbon, said shell part contains platinum (Pt) and said core part contains palladium (Pd), and when said shell part employs a two-layered structure, said shell part has: a first shell part containing palladium (Pd) formed to cover at least a part of a surface of said core part; and a second shell part containing platinum (Pt) formed to cover at least a part of a surface of said first shell part, and that said electrode catalyst concurrently fulfils conditions expressed by the following formulae and: X 1 / M ≤ 1.2 X 2 / M ≤ 47.0 and that (X2/M) in the formula exceeds 4.5, wherein in the formula and the formula, M represents an amount of substance (number of atoms) of one or more constituent metal elements of the shell part or the second shell part, as calculated using an electrochemical surface area (ECSA) based on a hydrogen desorption wave obtained by cyclic voltammetry, X1 represents an amount of substance (number of atoms) of bromine (Br), as calculated based on the content of a bromine (Br) species as measured by X-ray fluorescence spectrometry, and X2 represents an amount of substance (number of atoms) of chlorine (Cl), as calculated based on the content of a chlorine species, as measured by X-ray fluorescence spectrometry. a first shell part containing palladium (Pd) formed to cover at least a part of a surface of said core part; and a second shell part containing platinum (Pt) formed to cover at least a part of a surface of said first shell part, and that M represents an amount of substance (number of atoms) of one or more constituent metal elements of the shell part or the second shell part, as calculated using an electrochemical surface area (ECSA) based on a hydrogen desorption wave obtained by cyclic voltammetry, X1 represents an amount of substance (number of atoms) of bromine (Br), as calculated based on the content of a bromine (Br) species as measured by X-ray fluorescence spectrometry, and X2 represents an amount of substance (number of atoms) of chlorine (Cl), as calculated based on the content of a chlorine species, as measured by X-ray fluorescence spectrometry.