A mirror replacement device for a segmented mirror telescope including a segmented primary mirror in which a plurality of segment mirrors is removably arranged, the mirror replacement device comprising: - a gripping mechanism configured to grip the segment mirror from above with openable and closable gripping claw portions; - a fine drive mechanism having a lower end to which the gripping mechanism is fixed and, the fine drive mechanism being configured to change a position and a posture of the gripping mechanism by being driven along each of multiple axes; - a holder to hold the fine drive mechanism; - a lift mechanism to lift the segment mirror mounted and to be removed along a perpendicular axis direction of the segment mirror; - a first detector (111, 112) configured to detect a relative position and a relative posture between a comparison object being the gripping claw portions of the gripping mechanism or the segment mirror gripped by the gripping claw portions and a target object with which the comparison object is brought into contact; - a second detector configured to detect a bend of the holder or a bend of the fine drive mechanism including the bend of the holder; and - a mirror replacement controller configured to control and to drive the gripping mechanism, the fine drive mechanism, and the lift mechanism based on a detection signal output from the first detector (111, 112) and the second detector to replace the segment mirror, wherein the mirror replacement controller includes a determiner configured to determine whether or not the first detector (111,1 12) can successfully perform a measurement, and wherein, when the determiner determines that the measurement can be successfully performed, the drive control is performed based on the detection signal output from the first detector (111, 112), and when the determiner determines that the measurement cannot be successfully performed, the drive control is performed based on the detection signal output from the second detector. - a gripping mechanism configured to grip the segment mirror from above with openable and closable gripping claw portions; - a fine drive mechanism having a lower end to which the gripping mechanism is fixed and, the fine drive mechanism being configured to change a position and a posture of the gripping mechanism by being driven along each of multiple axes; - a holder to hold the fine drive mechanism; - a lift mechanism to lift the segment mirror mounted and to be removed along a perpendicular axis direction of the segment mirror; - a first detector (111, 112) configured to detect a relative position and a relative posture between a comparison object being the gripping claw portions of the gripping mechanism or the segment mirror gripped by the gripping claw portions and a target object with which the comparison object is brought into contact; - a second detector configured to detect a bend of the holder or a bend of the fine drive mechanism including the bend of the holder; and - a mirror replacement controller configured to control and to drive the gripping mechanism, the fine drive mechanism, and the lift mechanism based on a detection signal output from the first detector (111, 112) and the second detector to replace the segment mirror, wherein the mirror replacement controller includes a determiner configured to determine whether or not the first detector (111,1 12) can successfully perform a measurement, and wherein, when the determiner determines that the measurement can be successfully performed, the drive control is performed based on the detection signal output from the first detector (111, 112), and when the determiner determines that the measurement cannot be successfully performed, the drive control is performed based on the detection signal output from the second detector.