The increment of the DC current in the “Resonance Probe” is calculated from the second oder perturbation term of the electron distribution function, when it is expanded in power of a small amplitude of the AC voltage applied to the probe. The increment δj thus calculated well explains the experimental result (K. Takayama et al; Phys. Rev. Letters 5, 1960, 238), that is, δj retains a finite value for ω_o<ω_p, shows a resonant peak at ω_o= ω_p and goes down to zero for ω_o>ω_p, where ω_o and ω_p are the applied frequency and plasma frequency respectively, The result of calculation also shows that the ratio of the peak height to the value of δj at ω_o=ω is independent of the DC probe potenntial and given by
{(ω²+ν²) / (ω_p²+ν²)} |ε (ω) /ε (ω_p) |²,
the half width being give by 2ν, where ν is the effective collision frequency of the electron with neutral molecules and ε (ω) is the complex dielectric permittivity of plasma.