The impedance of a plane capacitor in a cold electron beam-electron plasma system is investigated theoretically. A radiation resistance results from excitations of two beam modes. It is found that the resistance becomes negative in a frequency range below an electron plasma frequency owing to the effect of a two-stream instability. As a special case, the impedance in a streaming collisionless plasma which includes no stationary plasma component is also discussed. This impedance is found to be very similar to the impedance for the stationary plasma having an equivalent collision frequency.