A production method of an electrode catalyst having a core-shell structure including 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 said production method comprises: a first step of retaining a liquid containing ultrapure water, a reductant and an electrode catalyst precursor under at least one stage of a temperature predetermined within a range of 10 to 95°C for a predetermined retention time, said electrode catalyst precursor being produced using a material containing chlorine (Cl) species, and exhibiting a chlorine (Cl) species concentration greater than 2,600 ppm when measured by X-ray fluorescence spectroscopy; a first drying step of the dispersion liquid obtained after said first step to obtain said electrode catalyst precursor powder contained in said liquid obtained after said first step; a second step of preparing a first liquid with said electrode catalyst precursor powder obtained after said first drying step being dispersed in ultrapure water, by adding ultrapure water to said electrode catalyst precursor powder obtained after said first drying step; a third step of preparing a second liquid by filtrating and washing the electrode catalyst precursor contained in said first liquid with ultrapure water, and then repeatedly performing washing until the electric conductivity (p) of the filtrate obtained after washing has become not higher than a first predetermined value selected from a range of not higher than 100 μS/cm when measured by JIS-standard testing method (JIS K0552), such that the electrode catalyst precursor, which is contained in said liquid exhibiting an electric conductivity (p) not higher than said first predetermined value, is dispersed in ultrapure water; and a second drying step of said second liquid. a first step of retaining a liquid containing ultrapure water, a reductant and an electrode catalyst precursor under at least one stage of a temperature predetermined within a range of 10 to 95°C for a predetermined retention time, said electrode catalyst precursor being produced using a material containing chlorine (Cl) species, and exhibiting a chlorine (Cl) species concentration greater than 2,600 ppm when measured by X-ray fluorescence spectroscopy; a first drying step of the dispersion liquid obtained after said first step to obtain said electrode catalyst precursor powder contained in said liquid obtained after said first step; a second step of preparing a first liquid with said electrode catalyst precursor powder obtained after said first drying step being dispersed in ultrapure water, by adding ultrapure water to said electrode catalyst precursor powder obtained after said first drying step; a third step of preparing a second liquid by filtrating and washing the electrode catalyst precursor contained in said first liquid with ultrapure water, and then repeatedly performing washing until the electric conductivity (p) of the filtrate obtained after washing has become not higher than a first predetermined value selected from a range of not higher than 100 μS/cm when measured by JIS-standard testing method (JIS K0552), such that the electrode catalyst precursor, which is contained in said liquid exhibiting an electric conductivity (p) not higher than said first predetermined value, is dispersed in ultrapure water; and a second drying step of said second liquid.