To smooth automatic collision prevention assist by enhancing visibility of a radar image and a tracking target, reducing an operation load of a user, and enabling an operation processing capacity effectively.

The echo of the target included in detection data is captured as a tracking object target from the detection data by a radar, and the target position changing following elapse of time is tracked. When the tracking object shown by marks Ma, Mb, Mc reaches a tracking release region A set backward from own ship shown by the mark MM, tracking on the target is automatically released. Hereby, the problem wherein a display content becomes complicated gradually can be avoided, and targets becoming unnecessary to be tracked are not required to be deleted one by one by manual operation, and the target required to be monitored can be tracked surely.