PURPOSE: To prevent the generation of the trouble of the breakage, etc. of paper by printing the same character twice at the same position of printing, printing the character again while displacing it at a minute pitch and bold-printing it in the serial impact-printer.

CONSTITUTION: When the bold printing data BD of one bit among the printing data PD of eight bits are at H, the A/N character or symbol by printing character data PD' is printed first by means of main controller 1, and printed again without moving a printing head. Space size signals SP2 and gate control signals GC are outputted while space strobe signals SPB are outputted, the printing head and a type wheel are moved only by minute space size l2 in the direction of printing, and the character is printed without turning the type wheel, and continuously printed again. A carriage is moved only by space size l3, which is smaller than predetermined space size l1 only by l2, and the printing head is shifted to the next printing operation.