An image data conversion method for converting the image data signal to the runlength signal through the process for reading once or several times a runlength code from a runlength conversion table for a given image data signal and a process for adding the pertinent runlength code when the same type of runlength code appears continuously. A character code/character pattern conversion apparatus which uses said image data conversion method, and provides, moreover, a multiplication device for multiplying character enlarging coefficient and the runlength code and a character interval inserting means which adds a character interval width code to the enlarged runlength code and outputs the input character code after converting it to the enlarged character pattern along with the interval code.