A plate attaching-detaching apparatus for attaching or detaching a plate to or from a plate-receiving metal frame of a sliding nozzle device, the plate attaching-detaching apparatus comprising: a robot arm; a plate holding device (101, 101A) mounted to a distal end of the robot arm, the plate holding device (101, 101A) comprising a plurality of holding members for holding the plate for the sliding nozzle device, widening and narrowing means to selectively widen and narrow a distance between the holding members, a pressing unit (9, 9A) for pressing a central region of the plate when the plate is held by the holding members, and a force sensor for detecting a force received by the holding members and/or the pressing unit (9, 9A) from the held plate; and a control unit for controlling operations of the robot arm and the plate holding device (101, 101A), wherein the control unit is configured to: during a plate detaching operation, controllably operate the robot arm so as to move the plate holding device (101, 101A) toward a plate attached to the plate-receiving metal frame; then when the force detected by the force sensor reaches a given threshold, controllably operate the robot arm so as to stop the movement of the plate holding device (101, 101A), and controllably operate the widening and narrowing means so as to narrow the distance between the holding members to hold the plate; and then controllably operate the robot arm so as to detach the held plate from the plate-receiving metal frame, the control unit being configured to: during a plate attaching operation, controllably operate the robot arm so as to move a plate held by the plate holding device (101, 101A) toward the plate-receiving metal frame; and, then when the force detected by the force sensor reaches a given threshold, controllably operate the robot arm so as to stop the movement of the plate holding device (101, 101A), and controllably operate the widening and narrowing means so as to widen the distance between the holding members to release the plate, thereby attaching the plate to the plate-receiving metal frame, and wherein the plate holding device further comprises a pressing mechanism for inhibiting a swingable metal frame openable and closable with respect to a fixed metal frame of the sliding nozzle device from being swung in a closing direction, the pressing mechanism being movable forwardly and backwardly with respect to the swingable metal frame and configured to contact an outer surface of the swingable metal frame during the plate detaching operation. a robot arm; a plate holding device (101, 101A) mounted to a distal end of the robot arm, the plate holding device (101, 101A) comprising a plurality of holding members for holding the plate for the sliding nozzle device, widening and narrowing means to selectively widen and narrow a distance between the holding members, a pressing unit (9, 9A) for pressing a central region of the plate when the plate is held by the holding members, and a force sensor for detecting a force received by the holding members and/or the pressing unit (9, 9A) from the held plate; and a control unit for controlling operations of the robot arm and the plate holding device (101, 101A), wherein the control unit is configured to: during a plate detaching operation, controllably operate the robot arm so as to move the plate holding device (101, 101A) toward a plate attached to the plate-receiving metal frame; then when the force detected by the force sensor reaches a given threshold, controllably operate the robot arm so as to stop the movement of the plate holding device (101, 101A), and controllably operate the widening and narrowing means so as to narrow the distance between the holding members to hold the plate; and then controllably operate the robot arm so as to detach the held plate from the plate-receiving metal frame, the control unit being configured to: during a plate attaching operation, controllably operate the robot arm so as to move a plate held by the plate holding device (101, 101A) toward the plate-receiving metal frame; and, then when the force detected by the force sensor reaches a given threshold, controllably operate the robot arm so as to stop the movement of the plate holding device (101, 101A), and controllably operate the widening and narrowing means so as to widen the distance between the holding members to release the plate, thereby attaching the plate to the plate-receiving metal frame, and wherein the plate holding device further comprises a pressing mechanism for inhibiting a swingable metal frame openable and closable with respect to a fixed metal frame of the sliding nozzle device from being swung in a closing direction, the pressing mechanism being movable forwardly and backwardly with respect to the swingable metal frame and configured to contact an outer surface of the swingable metal frame during the plate detaching operation.