Method for producing electrode catalyst, and electrode catalyst

An electrode catalyst in which a platinum alloy of platinum and a transition metal is loaded on a tin oxide carrier, wherein:

• in a diffraction pattern obtained by subjecting the electrode catalyst to an X-ray diffraction measurement, a diffraction angle 2θ at a peak of a (200) plane of the platinum alloy is from 46.5° to 48.0°;
• and
• a metallization rate of Sn, as defined by equation (1) below, in an analysis region on the electrode catalyst's surface and a vicinity of the surface as measured by X-ray photoelectron spectroscopy is 5% or less:
• metallization rate of Sn(%)=RSn-metal/(RSn-metal+RSn-oxide)×100 (Eq. 1),
• wherein RSn-metal is an area that Sn metal occupies in a spectrum ascribable to the Sn 3 d5/2 orbit as measured by X-ray photoelectron spectroscopy, and
• RSn-oxide is an area that Sn oxide occupies in a spectrum ascribable to the Sn 3 d5/2 orbit as measured by X-ray photoelectron spectroscopy.