A theoretical analysis is made of the resonance phenomena in the radio-frequency probe experiments of Takayama et al. The Boltzmann-Vlasov equation is solved under the action of an external rf electric field. The solution gives the resonance peak of the dc component of the electron current to the probe at the plasma frequency. For a partially ionized plasma, the peak-height δj and the half-width Δω1/2 are given by the following formulae. δj = j0 (1 / √2) (ωp / ν) (λd / L) √eV / κT (e δV / κT) I1 (e δV / κT), Δω1/2 = 2 ν. For fully ionized plasmas, they are determined as follows, δj = j0 (1 / √2) √eV / κT (e δV / κT) I1 (e δV / κT), Δω1/2 = 4 (λd / L)2 ωp. In the above expressions, T is the electron temperature, ωp is the electron plasma frequency, and λd is the Debye length. j0 is an eletron current density to the probe when no oscillating field is superposed on it, δV is the amplitude of the superposed rf voltage, I1(z) is the modified Bessel function of the first order. L is an effective penetration depth of the external field. V is the potential difference between the plasma space and the probe, and ν is the effective collision frequency of the electron with neutral molecules. The present theory confirms that the analysis of the resonance peak in the radio-frequency probe experiments is an effective method for the plasma diagnosis.