PURPOSE: To improve the printing quality in printing halftone-processing image without relying on a software processing by adding a simple circuitry and thereby generating a piece of black data to be two successive black data and correcting said data in case black data of only one dot which is sandwiched by white data in picture element data.

CONSTITUTION: A black data generating means is provided when black data of only one dot which is sandwiched by successive white data in picture element data, a piece of black data is generated to be two successive black data by the black data generating means, and said data are corrected to be transmitted. An AND circuit 11 and an OR circuit 12, for instance, are added between a shift register 2 and an interface drive 3. The AND circuit 11 at all time compares the picture data under serial transmission. And if white data is exists successively to black data of only one dot, the AND condition is satisfied, therefore, a piece of another black data is generated in front of the existing black data to be two successive black data, said data corrected to be transmitted. If the condition is not satisfied, black or white data are transmitted as they are by means of the OR circuit 12.