To provide a rack storage width adjusting device for simplifying storage width adjustment of a rack and achieving a small size and a plate material storage system having the rack storage width adjusting device for performing storage width adjustment of the rack and storage of plate materials in the rack in a continuous line and performing individually independent operation.

The storage width adjusting device 30 for the plate material storage rack comprises a moving device 41 for moving a movable part 70 forward or backward to a movable side plate 5, a supporting part 42 mounted on the movable part 70 for supporting the movable side plate 5 at its side face, a holding part 43 mounted on the movable part 70 for holding the movable side plate 5 together with the supporting part 42 and a first cylinder device 45 for putting an engagement part 44 engageable with a pivotal lever 32 in engagement with the pivotal lever 32 and moving the pivotal lever 32 to a lock or non-lock position. The plate material storage system 100 comprises the device 30 provided with a carrying device 40 for carrying the plate material storage rack 7 and a plate material storage device 50 for storing plate materials P in the plate material storage rack 7 with its storage width adjusted.