We measured the optogalvanic signal and discharge voltage-current(V-I) curve under the two different discharge conditions with different buffer gases, Ar, and Ne. When the Gd was used as a cathode material at low discharge current less than 10mA, a significant change was observed in the current-voltage curve. Time resolved optogalvanic signal measurement were measured by the diode laser of which wavelengths correspond to metastable transition line of these gases (Ar, Ne). From these measurements, we found that the characteristics of the V-I curve strongly depend on the Penning ionization process.