It is inferred from the phenomenological facts of tornado that tornado itself should have a driving force generating the updraft in the vortex of tornado. The driving force is supposed to be originated from the electric interaction between a storm cloud and a crustal conducting body underground. The electric interaction in tornado is considered to be similar to a lightning discharge in respect that the electric potential is reduced between a storm cloud and the ground, but the electric discharging process in tornado should be much slower than the lightning discharge and result in the atmospheric vortex formation of tornado. The mechanism of tornado formation is suggested, in which Townsend avalanche process is assumed inside the tornado vortex with a dangling conducting channel that is embedded in the funnel cloud of tornado. The strong updraft inside the vortex of tornado is produced by momentum diffusion process when the positively ionized air molecules move upward under the influence of electric field inside the vortex. With the mechanism of tornado formation, phenomenological expectations are followed.