A wafer carrier box contains a wafer carrier which holds at the same time, a plurality of thin sheets, such as magnetic disks, compact disks, silicon wafers for producing semiconductors, or glass bases for LCDs, for operations including conveying, storing and surface-processing the thin sheets. On wall surfaces of the wafer carrier box and/or wafer carrier are bar codes including processing histories and control directly written by a laser beam. The wafer carrier is formed of a material such as polypropylene containing 0.0001 to 0.5 percent by weight of Si or other material that acts as an exothermic body. The other material may be a catalytic substance normally included in production of the wafer carrier. Deterioration in identification capability, which may occur due to chemical liquids, is eliminated. Therefore damage to the bar code which provides lot identification is prevented and improved processing is achieved.