PURPOSE: To obtain a less-error monitoring system by varying a threshold value according to the amount of an error in tracking data, and using the best threshold value at each point of time and deciding on correlation among tracking data.

CONSTITUTION: A radar observed value Xi by radar devices 1A and 1B and an estimated value X'i by tracking processors 2A and 2B are sent to a monitoring device 10. calculating devices 3A and 3B for calculating an error index Ai send out error index data and a memory access circuit 4 accesses memories 5AW5D according to the combination of index values. In those memories 5AW5D, the best threshold values 1W4 are stored. In the figures, 6AW6D are registers for storing the best threshold values 1W4 read out of the memories, and the monitoring device 10 uses those threshold values to perform track correlation processing, and then outputs the result to an output equipment 11.