Plasma measuring method, plasma measuring device and storage medium

Provided is a technique capable of ascertaining the process condition of the boundary between electrically positive and negative plasma regions. In a vacuum chamber, one of the parameters of process conditions is stepwisely changed to generate a plasma under at least three process conditions. The parameters include a flow rate ratio between an electrically negative gas and an electrically positive gas, a pressure in the vacuum chamber and the magnitude of an energy supplied to the gases. Next, a voltage is applied to a Langmuir probe positioned in that plasma, and a current-voltage curve indicating the relationship between the applied voltage and the electric current to flow through the probe is acquired for each of the process conditions. On the basis of the current-voltage curve group acquired, the process conditions are determined for the boundary between the electrically positive and negative plasma regions.