Using a real-time formalism of thermal field theory, we derive the reaction-rate formula for neutrino-conversion (ν → ν ) process and νν̄ ′ annihilation process, which take place in an ultrarelativistic QED plasma consisting of electrons, positrons, and photons. Also derived is the formula for the inverse processes to the above ones. On the basis of the hard-thermal-loop resummation scheme, we i...