To make an intentional returning action to the two-wheel drive state unnecessary by providing a four-wheel drive temporary return means temporarily returning to the four-wheel drive state while the prescribed operation tool is operated in the two-wheel drive state.

A temporary return pedal 49 is integrally provided on a pedal plate 46 connected to second connecting rods 40L, 40R via a connecting member 47 or the like so that the second connecting rods 40L, 40R can be independently rotated by the action of the temporary return pedal 49. When the temporary return pedal 49 is depressed in the two-wheel drive state locking a drive switching lever 42 on a front lock groove 45a, the second connecting rods 40L, 40R are set to the nontensile state, and the travel drive state is temporarily returned to the four-wheel drive state. When a foot is released from the temporary return pedal 49, the second connecting rods 40L, 40R are pulled by a coil spring 48, and the two-wheel drive state is again maintained. No returning action to the two-wheel drive state is required, and the travel drive state can be temporarily returned to the four-wheel drive state easily.