PURPOSE: To reduce the vibration noise by blackening the supporting tube for supporting the rotary shaft connected with anode through bearing and the surface of radial gap motor facing with rotor thereby eliminating the gnawing of bearing.

CONSTITUTION: The anode 3 irradiated with electron from cathode 2 is rotated by rotary shaft 4 which is supported by supporting tube 6 through bearing 5. Rotor 7 of radial gap motor one end of which is connected to the rotary shaft 4 is provided at the outside of supporting tube 6, while the inner face of rotor 7 and the outer face of supporting tube 6 is blackened through proper processing. Since the transmission factor of radiated heat between rotor 7 and supporting tube 6 is increased to reduce the temperature difference, the gnawing of bearing is reduced while the gap in radial direction of bearing 5 is also reduced to reduce the vibration noise.