At least one of first and second hydraulic motor units that drive first and second wheels is of a variable displacement. A volume adjusting mechanism thereof is operated by an electric motor in accordance with manual operation or automatic operation based on difference in turning radius between the first and second wheels. A control device that controls the electric motor includes normal mode that operates the electric motor in accordance with the manual operation or the automatic operation and torque-up mode that fixes the variable displacement hydraulic motor to a first volume larger than a standard volume, and activates the torque-up mode at idling state in which a power source is in operating state and a pump-side volume adjusting mechanism is in neutral state and shifts the torque-up mode to the normal mode when a traveling speed reaches a first speed.