To correct a width of a thin line without influencing the whole of an image by a low costed structure.

An N-dot detection/positional detection circuit 20 detects an image of which the width is N dots based on the number of dots N representing the width of a vertical line to be corrected, and stores its position in a temporary storage 21. A relational operation circuit 22 compares a positional pixel value of a precedent line stored in the temporary storage 21 with the pixel value of the position of the present line detected with the N-dot detection/ positional detection circuit 20. When conformed, the line is detected as a vertical line. Further, a 45° slanting line is detected by comparing the pixels at the pixel positions in 45° slanting direction of the before and after lines. A pulse width adjusting circuit 23 adjusts the pulse width corresponding to positions of the vertical line and slanting line so as to be the same one as the width of a horizontal line according to characteristics of a printer engine 13.