A study has been made to clarify the nature of the thermal noise generated within the layer of the (Ba Sr) oxide coating of the cathode when a current flows through it, and to determine how much this noise affects the flicker noise. The results of this investigation show that (1) the excess noise generated by the flow of current is proportional to the square of the current flowing through the coating layers, (2) the frequency spectrum of the noise is inversely proportional to the frequency, (3) the noise is greatly affected by the activation of cathode, a slight decrease in activation causing a sudden increase of noise (4) the portion of the cathode flicker noise due to this excess noise may be neglected when the activation of the cathode is good, but when the activation is poor, it becomes considerably larger. The characteristic of (3) may be considered to give a clue to a clarification of the electron conductivity mechanism in the oxide cathode.