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 preparing a first liquid with an electrode catalyst precursor being dispersed in ultrapure water by adding said electrode catalyst precursor in powder form to the ultrapure water, thereby reslurrying said electrode catalyst precursor, said electrode catalyst precursor being produced using a material containing chlorine (Cl) species, and exhibiting a chlorine (Cl) species concentration not lower than 6,000 ppm when measured by X-ray fluorescence spectroscopy; and a second step of preparing a second liquid by dispersing an electrode catalyst precursor in ultrapure water, said electrode catalyst precursor being obtained by filtrating and washing said electrode catalyst precursor contained in said first liquid with ultrapure water, and then repeatedly performing washing until the electric conductivity (ρ) 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, wherein said electrode catalyst precursor used in said first step is a powder obtained by a pretreatment process comprising: a step of preparing a liquid with an electrode catalyst precursor being dispersed in ultrapure water by adding said electrode catalyst precursor to the ultrapure water, said electrode catalyst precursor being produced using a material containing chlorine (Cl) species, and exhibiting a chlorine (Cl) species concentration not lower than 6,000 ppm when measured by X-ray fluorescence spectroscopy; and having a core-shell structure; a step of preparing a liquid by dispersing an electrode catalyst precursor in ultrapure water, said electrode catalyst precursor being obtained by filtrating and washing said electrode catalyst precursor contained in said 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 predetermined value selected from a range of not higher than 100 μS/cm when measured by JIS-standard testing method JIS K0552; and a step of drying said liquid. a first step of preparing a first liquid with an electrode catalyst precursor being dispersed in ultrapure water by adding said electrode catalyst precursor in powder form to the ultrapure water, thereby reslurrying said electrode catalyst precursor, said electrode catalyst precursor being produced using a material containing chlorine (Cl) species, and exhibiting a chlorine (Cl) species concentration not lower than 6,000 ppm when measured by X-ray fluorescence spectroscopy; and a second step of preparing a second liquid by dispersing an electrode catalyst precursor in ultrapure water, said electrode catalyst precursor being obtained by filtrating and washing said electrode catalyst precursor contained in said first liquid with ultrapure water, and then repeatedly performing washing until the electric conductivity (ρ) 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, wherein said electrode catalyst precursor used in said first step is a powder obtained by a pretreatment process comprising: a step of preparing a liquid with an electrode catalyst precursor being dispersed in ultrapure water by adding said electrode catalyst precursor to the ultrapure water, said electrode catalyst precursor being produced using a material containing chlorine (Cl) species, and exhibiting a chlorine (Cl) species concentration not lower than 6,000 ppm when measured by X-ray fluorescence spectroscopy; and having a core-shell structure; a step of preparing a liquid by dispersing an electrode catalyst precursor in ultrapure water, said electrode catalyst precursor being obtained by filtrating and washing said electrode catalyst precursor contained in said 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 predetermined value selected from a range of not higher than 100 μS/cm when measured by JIS-standard testing method JIS K0552; and a step of drying said liquid. a step of preparing a liquid with an electrode catalyst precursor being dispersed in ultrapure water by adding said electrode catalyst precursor to the ultrapure water, said electrode catalyst precursor being produced using a material containing chlorine (Cl) species, and exhibiting a chlorine (Cl) species concentration not lower than 6,000 ppm when measured by X-ray fluorescence spectroscopy; and having a core-shell structure; a step of preparing a liquid by dispersing an electrode catalyst precursor in ultrapure water, said electrode catalyst precursor being obtained by filtrating and washing said electrode catalyst precursor contained in said 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 predetermined value selected from a range of not higher than 100 μS/cm when measured by JIS-standard testing method JIS K0552; and a step of drying said liquid.