An electrode catalyst is provided in which degradation of catalytic action over time is suppressed. The electrode catalyst includes a mesoporous carbon support having pores, and catalyst metal particles supported in at least some of the pores. The ratio r/R of the mean particle size r of the catalyst metal particles to the modal pore size R of the pores is from 0.01 to 0.6, and the mean particle size r is 6 nm or less. The modal pore size R of the pores is preferably from 2 to 50 nm.