PURPOSE: To enable a slip control to be started always in the optimum timing by detecting different operation information of two or more kinds and, when at least two or more of the operation informations satisfy a predetermined control start condition, starting a control of reducing a drive wheel speed.

CONSTITUTION: In the case of a vehicle provided with an engine having in an intake pipe 22 a main throttle valve 23 operated by an accelerator pedal and a subthrottle valve 24 controlled in its opening by a motor 24M with a control signal from a traction controller 15, it controls a drive wheel speed VF to be reduced by adjusting an opening of the subthrottle valve 24 in accordance with a slip amount DV calculated in accordance with a difference between a drive wheel speed VF and a non-drive wheel speed VB. Here different operation information of two or more kinds is detected, when at least two or more of the operational informations satisfy a control start condition combining an independently preset decision condition, a control is started of reducing the drive wheel speed VF.