PURPOSE: To decrease the DC component of a code string and to improve a low frequency cut-off characteristic by moving an RDS(Running Digital Sum) distribution of a transmission series in a direction so as to decrease the RDS in comparison with that of the original series.

CONSTITUTION: A 1 cell delay circuit 1, an inversion/noninversion circuit 2, a 1 cell RDS calculation circuit 4 and an RDS calculation circuit 5 are provided as means controlling the inversion or noninversion of bits in the unit of cells according to a prescribed rule evading the accumulation of the integration value of a code string are provided and an inversion indication bit addition circuit 3 is provided, which adds a bit indicating inversion or noninversion to each cell. Then the RDS distribution is moved to decrease the RDS. Thus, the DC component of the code string and the low frequency cut-off characteristic is improved.