To detect the abnormal state of a plugging device, and immediately inform a driver by arranging in a travelling motor, a rotation detector for detecting the rotational direction of a motor and a forward/backward travelling detector for detecting forward/backward travelling, and arranging in a controller, a plugging current detector for detecting plugging current.

When a driver operates plugging, a transistor 10A is set at a constant current value by the command value of an accelerator (the bringing down angle of a lever). Plugging current is led to flow into the armature 9A of a motor so as to generate a torque reverse to a travelling direction, and apply plugging damping. When the transistor 10A of a controller 10 gets out of order at the time of plugging operation, the plugging current may not be led to flow at last, therefore, the plugging abnormal state can be discriminated if the existence of the plugging current is detected by a plugging current detector 14. The travelling direction of a car is decided by a rotation detector 13, a travelling command is ordered by a forward/backward travelling detector 12, and the plugging abnormal state is informed to a driver by a warning device such as a lamp.