A driving-torque difference value, an inertial-force difference value of the vehicle, and an inertial-force change-amount difference value of the vehicle are calculated. Subsequently, a first determination coefficient by which the inertial-force difference value is to be multiplied or a second determination coefficient by which the driving-torque difference value is to be multiplied is estimated on the basis of a state equation having the inertial-force difference value as a state variable and the driving-torque difference value as an input variable. Subsequently, a road-surface condition is determined on the basis of a comparison between a threshold value and the first determination coefficient or the second determination coefficient.