PURPOSE: To achieve a common control code conversion circuit with simple circuit constitution, by providing a mode switching function in a code converter.

CONSTITUTION: A code converter 6 which outputs an information with code conversion through the input of a digital information and a mode selection information, and a mode selection information for the next digital information, and a delay circuit which delays the mode selection information outputted from the converter 6 and feeds it back to the converter 6, are provided. Relating to the code conversion of weight 1 for m1 and weight 3 for m2 of mode selecting information m1, m2, the discrimination is made for the odd or even number order, depending if m1, m2 are 0 or 1 and conversion is made with different mode. Mode selecting information m1', m2' to be outputted is the inversion of the m1 as to the code of weight 1 and the inversion of m2 as to the code of weight 3. The transmission line code having uniform probability of appearance of 0, 1 can be outputted by making the mode selection continuously.