Proposed are non-invasive diagnostic method and apparatus for plasma processes in which plasma processes can be monitored in real time by measuring a surface wave resonance frequency generated in plasma or a sheath on the basis of a surface wave resonance principle. The diagnostic method may include a step (S10) of installing at least one probe on one side of an electrostatic chuck (ESC) or on an inner wall of a chamber, a step (S20) of emitting a high frequency onto the plasma or the sheath by the probe, and a step (S30) of detecting a frequency reflected from the plasma or the sheath by the probe. In addition, the diagnostic method may include the step (S40) of extracting a reflection spectrum, a step (S50) of extracting the surface wave resonance frequency, and a step (S60) of extracting electron density or uniformity of the plasma.