Origin of the electric dipole at high-k/SiO2 interfaces was investigated by classical molecular dynamics simulation. Both directions of the dipole are successfully reproduced with a simple two-body rigid ion model. The direction and magnitude of the dipole are determined by two opposing tendencies; oxygen ion migration from higher density oxide side to lower one, and the migration of metal cations in high-k toward SIO2 side to form a silicate layer at the interface. The driving force of oxygen ion migration is the core-to-core repulsive interaction between oxygen ions. This is agreeing with the oxygen-density-difference-accommodation model proposed by Kita and Toriumi. The driving force of the cation migration is the energy gain to form a stable silicate phase. Thus, the migration of metal cations must be taken into account as well as the migration of oxygen ions to comprehensively explain the mechanics of the dipole layer formation.