A plurality of energy efficiencies of an electric motor which are determined by characteristics of the electric motor are set according to the revolution speed and the driving force of the electric motor, and among the plurality of energy efficiencies, a plurality of energy efficiency groups having the equivalent energy efficiency are prepared. The energy efficiency for each transmission gear of a first input shaft is determined on the basis of a travelling speed and a required driving force of a vehicle. To which group among the plurality of energy efficiency groups that each determined energy efficiency is pertained is determined, and a transmission gear corresponding to the energy efficiency group having the highest energy efficiency among the determined energy efficiency groups is selected.