Voltage, plasma potential and electron temperature of a medium vacuum arc ignited between Ti cathode and disk stainless steel anode were measured in various ambient gases; H-2, N-2, He and Ar. Gap length was varied from 75 to 450 mm. The result showed that the voltage increased with gap length and gas pressure, and that the voltages in H-2 and N-2 were higher than those in He and At-. The influence of the gas species on the voltage was explained on the basis of: the mean free path; ionization and excitation processes of the gas; and ambipolar diffusion,