An electrode catalyst ink composition which includes metal oxide-based electrode catalyst particles, an electrolyte, and a mixed liquid medium, wherein the mixed liquid medium contains 40 to 85% by mass of water; 5 to 30% by mass of an aqueous solvent (A) that has an evaporation rate of 2.0 or lower when the evaporation rate of water at 25° C. is 1, and a solubility parameter (SP value) of not less than 9; and 10 to 30% by mass of a monoalcohol (B) that has an evaporation rate of higher than 2.0 when the evaporation rate of water at 25° C. is 1, and not more than 3 carbon atoms, provided that the total amount of the mixed liquid medium is 100% by mass.