A collision avoidance system for a mobile unit includes an image capturing unit for capturing an image of an environment surrounding a mobile unit. Motion vector is calculated based on the captured image. Collision probability is calculated based on the motion vectors of the image. The system includes a plurality of receptive field units that are modeled on the optic lobe cells of the flies. Each of the receptive field units includes a filter producing an excitatory response to the motion vector diverging from the center of the receptive field in the central area of the receptive field and producing an inhibitory response to the motion vector converging toward the center of the receptive field in the areas around the central area. The outputs of the receptive field units are compared to determine a direction in which the obstacle approaches the mobile unit. The mobile unit moves in a direction to avoid collision.