The effect of secondary electron emission from the cathode on the ionization current multiplication in the non-self-sustaining Townsend condition is investigated adopting the frequency response method. Measurement is carried out in N2 gas and the property of N2(A3SIGMA(u)+) metastable molecules is analysed discriminating it from other secondary processes due to ions and photons in the frequency domain. The results are consistent with the data of transient current analyses. The frequency response method may be valid for the detailed analyses of the secondary processes in the total ionization current system.