To provide interchangability of an input interface for printing characters of a line dot type thermal printer with a dot impact type printer.

This line dot type thermal printer has a character code for determining printing characters, the first RAM for storing data on height of characters for determining the number of lines by starting development of characters and a modification code for determining necessity of development based on the kind and size of characters and the second RAM for storing data of developed one dot line for one width of printed characters. Then, development is successively performed from the front figure by judging undevelopment/during developing and on undeveloped characters with low height of character, the line counter is reduced and developing processing is not performed until it becomes [=0] and on characters during developing, the condition of modification is operated only in case of different kind of mode and a method for developing pattern of characters is constituted in such a way that the condition of modification is not newly operated when the mode is the same kind and said method for developing is applied.