A peripheral aircraft selection means maps a region in a 3D space defined by a start point of period information of the aircraft of interest, a point having the coordinate values of passage position coordinates at an end point of the period information and a lower limit arrival time, and a point having the coordinate values of passage position coordinates at the end point of the period information and an upper limit arrival time onto a 2D plane by use of a first projection matrix. The peripheral aircraft selection means then selects a peripheral aircraft which can abnormally near to the aircraft of interest by determining how the resultant region crosses with a circle whose center is a passage position of the peripheral aircraft and whose radius is a determination standard threshold as to whether abnormal nearing will occur.