PURPOSE: To clearly discriminate a space for an automobile, by respectively counting clocks having different frequencies by a first and a second counters, and discriminating the space by measuring the distance and speed of an object through laser.

CONSTITUTION: Emitted and transmitted laser beam through a glass plate 5 and the receiving beam due to reflection from an object are received by a photoelectric converter 14 and a gate 18 is controlled by BPF15, a wave form shaping circuit 16 for outputting a rectangular wave corresponding to the Doppler signal of the received light and FF17 while low frequency is counted by a counter 20 to determine and objective speed. On the other hand, a counter 24 counts high speed clock during a rectangular wave period corresponding to the period between light transmission and reception to determine the distance from an object. The speed and the distance due to the counters 20, 24 are processed by a computer 9 and rapidly and effectively utilized to clearly discriminate the space for an automobile.