Abstract: The thermal printer in which an irregular character pattern such a bar code or an OCR (optical character recognizer) character which are different in length and width from the standard font patterns are modified into bit patterns having the same size as that of the standard font patterns. These bit patterns are stored along with restoring data for restoring the bar codes with restoring data for restoring the bar codes and OCR characters to their original pattern for output to a printer. The thermal printer is responsive to a heating element driving pulse signal which is applied once during the data cycle when an ordinary character or graph is printed and twice during data cycle when a bar code is printed. The second pulse per data cycle is selectively applied to the heating element for each irregular character such that the thermal printer is heated to higher temperatures, thereby increasing the diameter of each dot to be printed so that the ruggedness of the contour line of the bar code is minimized.