PURPOSE: To recognize a ground relief height in a flight area while a missile is flying.

CONSTITUTION: Output of an altimeter 11 of a missile 3 itself is taken into a memory by a data take-in part 21 of a computer at a fixed time interval for a predetermined time ts, and a standard deviation of the taken data is computed by a standard deviation computing part 22. By referring to a chart 25, of correlation between a ground relief height and the standard deviation, stored in advance in a chart storage part 23, a ground relief height X corresponding to the computed standard deviation is found by a ground relief height judging part 24.