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 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 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 , 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 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 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 , 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 . <EOS>