A probe (6) is brought into contact with a plasma produced by ionizing Ar gas, a saturation current (Ies2) at which current flowing through the probe is saturated when the potential of the probe is changed in a potential region where the potential of the probe is higher than a ground potential, and a saturation current (Iis2) at which current flowing through the probe is saturated when the potential of the probe is changed in a potential region where the potential of the probe is lower than the ground potential. Similarly, saturation currents (Ies2, Iis2) are measured by bringing the probe (6) into contact with a plasma produced by ionizing a mixed gas containing Ar gas and a process gas, such as C4F8 gas, and changing the potential of the probe (6). The negative ion density of the plasma produced by ionizing C4F8 gas is determined by using saturation current ratios (Iis1/Iis2, Ies1/Ies2).