Catalyst, electrode catalyst layer using same, membrane electrode assembly and fuel cell

[Problem] To provide an electrode catalyst for fuel cells, which is increased in the rate of catalyst metal utilization and is capable of reducing the production cost of a fuel cell. [Solution] A catalyst which is composed of a catalyst carrier and a catalyst metal supported by the catalyst carrier, and which is characterized in that the BET specific surface area per carrier weight is more than 1200 m 2 per gram of the carrier and the amount of acidic groups per carrier weight is not less than 0.7 mmol per gram of the carrier.