Provided is a field emission type electron gun having a means of removing amorphous carbon existing at a tip part of fibrous carbon substance without destroying its tip part, and also to provide its operation method. The field emission type electron gun solving the problems of the invention comprises: a field emission type cathode constituted of a single fibrous carbon substance and a conductive base material supporting it; a drawing device having electron emitted in the field; and an accelerator accelerating electron, which is provided with: a means of heating the field emission type cathode; and a means of impressing voltage of a polarity which does not allow the field emission type cathode to emit electron in the field.