Provided is a plasma diagnostic apparatus having a probe unit, which is inserted into a plasma or disposed at boundary of a plasma, the apparatus including: a signal supplying unit having a signal supplying source; a current detecting/voltage converting unit for applying a periodic voltage signal applied from the signal supplying unit to the probe unit, detecting the magnitude of the current flowing through the probe unit, and converting the detected current into a voltage; and a by-frequency measurement unit for computing the magnitude and phase of individual frequency components of the current flowing through the probe unit by receiving the voltage output from the current detecting/voltage converting unit as an input.