A plasma generating unit for a process monitoring device includes a hollow first electrode extending in a length direction and a second electrode extending in the length direction and positioned within and displaced from the first electrode with a distance therebetween. The first electrode has an inner diameter and the second electrode has an outer diameter selected to vary the distance between the electrodes in the length direction so that the plasma generating unit generates a plasma by ionizing a gas flowing between the electrodes at a different position in the length direction based on a pressure of the gas.