PURPOSE: To realize an automatic control method for a thermal electron cathode in a scanning type electron microscope which can automatically set the heating current of the thermal electron cathode accurately at an optimum condition.

CONSTITUTION: Current to be fed to a thermal electron cathode 2 from a heating power supply 5 is set at a value little less than an estimated optimum value Ia by controlling a computer 20. Heating current is then graduay increased, secondary electrons emitted from a specimen 13 is detected during that time, and detected signals are converted into digital signals bar an A/D converter 19, so that the resultant is fed to the computer 20. The computer 20 then monitors the change of the intensity of the secondary electron signals, and starts controlling the heating power supply 5 at the time when the intensity is saturated, so that action for increasing current is suspended, and the control of the heating power supply 5 is so executed that current is to be heating current at the time when the secondary electron signal intensity is saturated.