[Object] In the control of electron beam focusing of a pierce-type electron gun, any influences from the space charge effect and space charge neutralizing action within the electron gun are eliminated to attain complete control of an electron beam.[Solving Means] Feedback control of the pressure within the electron gun is performed by directly measuring temperature at an internal of the pierce-type electron gun. It is desirable that locations where the direct measurement of the temperature at the internal of the electron gun is performed are an anode (39) and a flow register (43). Further, the direct measurement can be performed at any one of a ring, an aperture and an exhaust pipe provided at an outlet or an inlet of any one of a cathode chamber (31), an intermediate chamber, and a scanning chamber (33). Accordingly, all of stabilization of beam producing area (optimized design of electron gun itself), stabilization of beam transporting portion and stabilization of beam using portion have become appropriate.