To provide a plasma forming device in which plasma and a droplet traveling from an arc discharge part are surely separated, the plasma is led to a plasma progress passage, the droplet is surely sampled in a droplet sampling part, and an approach of the droplet into the plasma progressive passage is prevented.

The plasma formation device is made to generate the plasma by a vacuum arc discharge of the arc discharge part 1, and the droplet by-produced from the cathode at the time of plasma generation is captured to the droplet capturing part 3. The main advancing passage 2 in which the plasma and the droplet advance in a mixed condition, the main advancing passage 2 is branched into the droplet advancing passage 4 in which the droplet advances and a first plasma advancing passage 5 in which the plasma advances, a restricting plate 7 in order to restrict the advancing of the plasma and the droplet at a middle part of the main advancing part 2 is installed, and a slanting wall 8 which reflects the droplet passing through the restricting plate 7 toward the droplet capturing part 3 is installed in the middle part of the droplet advancing passage 4, and formed at a position beyond the first plasma advancing passage 5.