PURPOSE: To eliminate a white line by dividing a heating unit into plural blocks so that dots on a boundary can be shared in two blocks.

CONSTITUTION: Heating dots D1∼D10 are separated into the 1st block B1 for consisting of the dots D1∼D5 and the 2nd block B2 for consisting of the dots D5∼D10 by overlapping the dot D5 fixedly. Two strobe signals SS corresponding to the number of blocks are obtained from a strobe signal generator circuit 11. A level data LD is multiplied by the strobe signals SS through an AND gate 10, after which a block level signal BLD is inputted to a thermal head TH, and a picture element corresponding to the block B1 is printed. Then the strobe signal SS is changed, and a picture element of the block B2 is printed, thereby printing picture elements of one line.