NOVELTY - An ink comprises electrode catalyst, electroconductive material, proton-conductive material and solvent. The electroconductive material contains transition metal element, carbon, nitrogen and oxygen as constituent elements. The atomic ratio of number of atoms of transition metal element, carbon, nitrogen and oxygen is 1:(0.5-7, excluding 0.5):(0.01-2, excluding 0.01):(0.1-3, excluding 0.1). The mass ratio (A/B) of content (A) of electrode catalyst and content (B) of electroconductive material is 1-6.    USE - Ink is used for forming electrode catalyst layer for membrane electrode assembly in fuel cell e.g. polymer electrolyte fuel cell (all claimed).    ADVANTAGE - The ink-containing electrode catalyst layer has high efficiency and excellent durability, and is economical and formed efficiently. The fuel cell using ink has excellent power generation characteristics.    DETAILED DESCRIPTION - An ink comprises electrode catalyst, electro-conductive material, proton-conductive material and solvent. The electro-conductive material contains transition metal element, carbon, nitrogen and oxygen as constituent elements. The atomic ratio of number of atoms of transition metal element, carbon, nitrogen and oxygen is 1:(0.5-7, excluding 0.5):(0.01-2, excluding 0.01):(0.1-3, excluding 0.1). When a transition metal element is multiple types, the number of atoms of transition metal element is total number of atoms of transition metal elements. The mass ratio (A/B) of content (A) of electrode catalyst and content (B) of electro-conductive material is 1-6. The mass ratio (D/C) of total content (C) of electrode catalyst and electro-conductive material and content (D) of proton-conductive material is 0.1-0.9. INDEPENDENT CLAIMS are included for the following:    (1) electrode catalyst layer, which is formed using ink;    (2) electrode, which has electrode catalyst layer and gas diffusion layer; and    (3) membrane electrode assembly, which has electrolytic membrane positioned between anode and cathode, in which electrode is used as cathode.