A driving-force transmitting device includes a first disengaging mechanism that disengages a driving force from an engine to a driving-force transmitting unit and a second disengaging mechanism having a synchronizing mechanism that disengages the driving force from a driving-force transmitting unit to a right-rear-wheel driving shaft. A change gear ratio between front wheels and rear wheels is constructed so that a driving-force transmitting unit side of the first disengaging mechanism rotates at a speed higher than that of an opposing side at the time of completion of synchronization of the second disengaging mechanism. In switching from a two-wheel drive mode to a four-wheel drive mode, synchronization of the second disengaging mechanism first starts to increase the rotation speed of the driving-force transmitting unit, and next connects the first disengaging mechanism when a rotation speed of the driving-force transmitting unit side of the first disengaging mechanism matches that of an opposing side.