Primary electrons impinge upon an emitter section of an electron emitter for causing emission of the secondary electrons. The secondary electrons are outputted from the electron emitter. The secondary electrons emitted from the emitter section are accelerated in an electric field applied to the emitter section for generating an electron beam. The electron emitter has the emitter section having a plate shape, a cathode electrode formed on a front surface of the emitter section, an anode electrode formed on a back surface of the emitter section. A drive voltage from a pulse generation source is applied between the cathode electrode and the anode electrode through a resistor. The anode electrode is connected to GND. A collector electrode is provided above the cathode electrode. A bias voltage is applied to the collector electrode.