Surge arrester with a spark gap has been installed for reducing electrical failures in Japanese distribution system. The top and the bottom of a ceramic spacer for sustaining an electrode gap in a surge arrester contact with electrodes, of which the electric field becomes high. Therefore, high electric field region such as the triple junction point becomes a source to supply electrons to a spark-gap. In this study, we made some electrodes with a small step edge on an inclined plane of an electrode. The electric field around the small step edge became high, and we aimed that the small step edge functioned as if it were a triple junction points for a conventional spark gap. Based on measurements of V-t characteristic and the number of electrons per unit time, we investigated that the artificially formed high electric field region lower impulse spark-over voltage and that a spark-over voltage occurs within a duration of 0.4-1.2µs when a positive impulse voltage of 25-42kV with 1.2/50µs is applied. The results will contribute on a further stabilization of discharge and increasing a degree of freedom for design of a spark gap.