A flat display device comprises a vacuum envelope constituted by a back base plate consisting of an elastic metal plate and a display panel and accommodating a thermionic cathode structure and a plurality of electrode structures for controlling the electron beams emitted from the thermionic cathode structure. The thermionic cathode structure is divided into a plurality of sections each including a plurality of coiled thermionic cathode heaters connected in parallel and arranged such that each corresponds to each of picture element regions provided on the back side of the display panel. Voltage supply terminals for supplying power to the parallel coiled heaters in each section are led out through the back base plate via insulating members.
