The present invention addresses the problem of providing: a core-shell catalyst capable of achieving, when evaluated for a fuel cell, the catalytic activity anticipated from the catalyst activity value obtained using a rotating disc electrode (RDE); and a method for activating a core-shell catalyst in said manner. The present invention relates to a fuel cell catalyst, which is an electrode catalyst having a core-shell structure and is characterized in that at least 99% of the core member is covered by the shell member and the halogen content is not more than 5000 ppm. The present invention also relates to a method for activating said core-shell catalyst, the method comprising: a process for dispersing the core-shell catalyst in a dispersion solvent; a process for separating impurities from said core-shell catalyst by blowing a gas that has reducing properties or a mixed gas comprising same into said dispersion solvent; and a process for removing said impurities.