Provided is a copy pair monitoring method which is for a storage system having at least one host computer, at least one storage subsystem, and a management computer, the storage subsystem including volumes storing data requested by the host computer to be written, the management computer being accessible to the host computer and the storage subsystem. In one embodiment, an IC includes a placement grid which includes a plurality of cells. The illumination of the light source is focused at an inclined angle to the surface of the sheet stack, substantially parallel to the lifting element row.