PURPOSE: To detect a possible collision in its early stage and eliminate all collisions between all objects by finding the remaining time that one object requires to collide against the front and rear limit tracks of each of plural other objects.

CONSTITUTION: A two-dimensional graph generated on a processor control display represents the track of one aircraft ACi and the front and rear limit tracks of another aircraft ACk. Those limit tracks are calculated by encircling the one aircraft ACi in a parallelogram 11 encircling an scheduled protection flight space and isolating the aircraft AC1 from a parallelogram corresponding to the remaining aircraft ACk. Consequently, while a possible collision is detected in its early stage, all collisions between all objects are eliminated and a collision can be evaded by changing the minimum tracks of relative objects.