To achieve the homogeneity of electron beam intensity by independently controlling the voltages of a plurality of heaters for applying electron impacts to the back face of a cathode having an electron emission face.

Eight heaters 5, for example, are radially provided at uniform intervals below a back face corresponding to a ring-like electron emission face 6 of a cathode 3, and a DC heating power supply 7 and a bombard voltage 8 are connected to the heaters 5. Electro-bombards from the heaters 5 hit the back face, and the generated heat heats the electron emission face 6. The electron intensity distribution displayed on a CRT is observed, heaters 5 requiring voltage control are specified among a plurality of heaters 5, bombard voltage values are checked for the optimum intensity distribution, and the heaters 5 are independently controlled via an interface 9 by the instruction from a CPU connected to the interface 9. The number of the heaters 5 is optionally determined according to the size of the cathode 3.