The modification shown in FIGS. 14 and 15 differs from that shown in that the maximum specifying circuit 39 shown in FIG. 12 is removed and a circuit including a specifying circuit 40 and an ambiquous region setting circuit 41 is used, instead and also in that a reinspection determining circuit 42 including a defective determining VD number circuit 43, a detected bit counting circuit 44 and a dete