The mechanism of radiation of plasma noise inherent to arc discharge is studied experimentally. Intense electromagnetic radiation of random noise character is observed at the frequency of plasma oscillation of the ion sheath formed at the cathode drop. Maximum radiation occurs when the external circuit connected to the cathode is in resonance with the plasma frequency. The oscillating current in the external circuit is concluded to be generated by the periodic electron emission from the cathode caused by small perturbation of potential gradient at the cathode surface as a result of the variation in the ion sheath potential. This conclusion is supported by several experiments under various air pressures, electrode materials, current densities, etc.