[Problem] A two-wheel→four-wheel drive switching for a traction-transmitting part time four-wheel drive vehicle in which a gear clutch for separating one of auxiliary driving wheels from a drive train is equipped can be carried out under a shock reduction.[Means for solving problem] In a case where a two-wheel→four-wheel drive switching request occurs during a traveling of the vehicle (step S11 and step S12), an operation of switching a traction-transmitting transfer from a non-transmitting state to a transmitting state is preceded (step S18 and step S22) and, thereafter, the gear clutch is engaged (step S21 and step S22) when a revolution speed difference ΔVw between main and auxiliary driving wheels is 0