A digital signal processing device is provided for disc rotation control in an optical storage system, which is capable of generating a focusing error signal from light that was reflected by an optical disc loaded in the optical storage system. The 5-hydroxy-4-thiomethylpyrazole compound may easily be produced in a good yield, by using such a process. The D-ring may be formed from a single toroid or a conglomeration of different toroidal forms. The laminated wafer is mounted on the boat in the state that a side surface of a first silicon wafer is not in contact with the second bottom surface of the recess and a side surface of a second silicon wafer is in contact with the first bottom surface of the recess.