Before performing additive manufacturing of an article to be formed, a first scale plate is scanned in a first scanning speed so that a trace of the electron beam is depicted. An electric current value through a focusing coil with which the trace of the electron beam becomes narrowest is found and set as a melting electric current value. Then, a second scale plate is scanned similarly in the first scanning speed so that a trace of the electron beam is depicted. An electric current value through the focusing coil with which the trace of the electron beam cannot be seen is found and set as a preheating focusing electric current value. Dispersed metal powder is scanned with electron beam of the preheating electric current value as set before in a second scanning speed 20 to 30 times of the first scanning speed. Thereafter, the additive manufacturing is performed.
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