Provided is a mirror replacement device (1) for a segmented mirror telescope, which is capable of positioning a segment mirror (3) with high accuracy. The mirror replacement device includes a gripping mechanism (6) to grip the segment mirror (3), a fine drive mechanism (7) to change a position and a posture of the gripping mechanism (6), a lift mechanism (4) for the segment mirror, a first detector (111, 112) to detect a relative position and a relative posture between a comparison object and a target object, a second detector (110) to detect a bend of the fine drive mechanism, and a mirror replacement controller to replace the segment mirror based on detection signals output from the above-mentioned detectors. The mirror replacement controller includes a determiner to determine whether or not the first detector (111, 112) can successfully perform a measurement. When it is determined that the measurement can be successfully performed, the control is performed based on the detection signal output from the first detector. When it is determined that the measurement cannot be successfully performed, the control is performed based on the detection signal output from the second detector.
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