An innovative technique to evaluate electric potential difference of triboelectricity between both dielectric surfaces was developed. Electric-discharge luminescence due to electrification during friction between a spherical diamond surface and a flat quartz one was observed using a microscope under various pressures of N2 gas. Two-dimensional spatial distribution of the luminescence around a contact shows a strong luminescence immediately after the friction and ring pattern of luminescence outside of friction area. The distribution of the strong luminescence along sliding direction, i.e., breakdown characteristics depending on gap distance, was well explained by a semi-empirical equation of discharge-current and gap distance. The potential difference between the both sides was also evaluated. Using the relation between a potential difference and a charge density in parallel-plate capacitor, surface charge density in the discharge area was calculated.