4 it is determined that the obstruction ahead is a vehicle then when no signal is sent by any one of the optical sensors it is determined that the obstruction ahead is not the vehicle furthermore the detection of a logical sum using a plurality of sensors in this way leads to the effect that the trailing vehicle is effectively moved forward in addition even if the plurality of vehicle determination sensors 7 a and 7 b of fig4 and logic such as an or condition are used there is the effect that the trailing vehicle is effectively moved forward note that as specific embodiments of the above described long range detection sensor and the short range detection sensor there are conical beam sensors which output long conical beams beam scanning sensors which scans beams and the like fig5 is a view showing one example of using conical beam sensors as sensors s 1 and s 2 of fig1 to detect the upper end and lower end fig5 a is a view showing the detection range when the vehicle is seen from the side surface and fig5 b is a view showing the detection range when the vehicle is shown upper side thus when the conical beam sensors are used as sensors s 1 and s 2 the detecting range has a conical shape expanding widely in a width direction and thinly in a height direction moreover fig7 shows one example of a detection range in this figure the conical beam sensors shown in fig5 are arranged near the outer periphery of the vehicle 41 also fig6 a and fig6 b are.