The left and right rear wheels 16B are controlled independently, and the traveling direction of the vehicle 10 is changed (turned) with the speed difference thereof. In other words, conventionally, it was necessary to provide a steering wheel, and the passenger was required to constantly operate such steering wheel. Nevertheless, since merely a designation of the turning direction by previously moving the operation stick 92 to the left or right will enable an automatic turn at an optimum curvature, the passenger is freed from complicated steering operations. Moreover, since the left and right independent control is implemented with PLL control, a highly precise and highly responsive turn is realized.