The four tables P, Q, R and S contained in an ROM 2 for 16-24 modulation have a total number of data in which the continuing bits are 2 bits, that is not larger than i bits among the 24 bits, so that the number of continuing bits having the same code is from 2 to 8 bits, so as to have the same characteristics as the (1, 7) code, and to stably lock the PLL. A comparator/selector circuit 5 selects an optimum table out of the tables P, Q, R and S.
