An electron gun (100, 201, 301) comprising: a cathode (111, 211, 311) for emitting thermoelectrons; a Wehnelt electrode (112, 212, 312) for focusing the thermoelectrons and to which a bias voltage is applied upon operation; a control electrode (113, 313) for extracting the thermoelectrons from a distal end of said cathode (111, 211, 311); an anode (114, 214, 314) for accelerating the thermoelectrons, and for irradiating a powder with the thermoelectrons as an electron beam (120, 220, 370); wherein the electron gun (100, 201, 301) is characterized in that it further comprises an optimum condition collection controller (105, 341) configured - to determine a degree of wear of said cathode (111, 211, 311) based on an emission current of said electron gun (100, 201, 301) including said cathode (111, 211, 311), said Wehnelt electrode (112, 212, 312), said control electrode (113, 313), and said anode (114, 214, 314), and - to execute fine adjustment for changing only the bias voltage without changing the control electrode voltage of said electron gun (100, 201, 301) or readjustment for changing the bias voltage and the control electrode voltage of said electron gun (100, 201, 301) based on the determination result and - to automatically decide a combination of the bias voltage and the control electrode voltage at which a brightness of the electron beam (120, 220, 370) reaches a peak. a cathode (111, 211, 311) for emitting thermoelectrons; a Wehnelt electrode (112, 212, 312) for focusing the thermoelectrons and to which a bias voltage is applied upon operation; a control electrode (113, 313) for extracting the thermoelectrons from a distal end of said cathode (111, 211, 311); an anode (114, 214, 314) for accelerating the thermoelectrons, and for irradiating a powder with the thermoelectrons as an electron beam (120, 220, 370); wherein the electron gun (100, 201, 301) is characterized in that it further comprises an optimum condition collection controller (105, 341) configured - to determine a degree of wear of said cathode (111, 211, 311) based on an emission current of said electron gun (100, 201, 301) including said cathode (111, 211, 311), said Wehnelt electrode (112, 212, 312), said control electrode (113, 313), and said anode (114, 214, 314), and - to execute fine adjustment for changing only the bias voltage without changing the control electrode voltage of said electron gun (100, 201, 301) or readjustment for changing the bias voltage and the control electrode voltage of said electron gun (100, 201, 301) based on the determination result and - to automatically decide a combination of the bias voltage and the control electrode voltage at which a brightness of the electron beam (120, 220, 370) reaches a peak. - to determine a degree of wear of said cathode (111, 211, 311) based on an emission current of said electron gun (100, 201, 301) including said cathode (111, 211, 311), said Wehnelt electrode (112, 212, 312), said control electrode (113, 313), and said anode (114, 214, 314), and - to execute fine adjustment for changing only the bias voltage without changing the control electrode voltage of said electron gun (100, 201, 301) or readjustment for changing the bias voltage and the control electrode voltage of said electron gun (100, 201, 301) based on the determination result and - to automatically decide a combination of the bias voltage and the control electrode voltage at which a brightness of the electron beam (120, 220, 370) reaches a peak.