PURPOSE: To make proper character spacing and print with high quality, by a method wherein a total width of a group of characters contained in a predetermined line is calculated from the widths of inputted characters, then the maximum line length is detected therefrom, and is made to be a printing width for one line.

CONSTITUTION: When character code data W are inputted through a keyboard or the like, the width of the character is determined by a character width table 1, and is added in an adder 3. When an input of a word end indicating character (a space, a hyphen or the like) is supplied to a detecting circuit 2 for the word end indicating character, the content (W1) of the adder 3 is added in an adder 6. At this moment, the adder 3 is reset. When it is discriminated by a line end detecting circuit 5 that transfer to the next line should be conducted, a maximum line length detecting circuit 8 receives the content (W) of the adder 6, and stores it. At this moment, the adder 6 is reset. These operations are performed until the last line in a page is detected by a last line detecting circuit 7. When the last line is detected, the detecting circuit 8 detects the maximum line length (MAX) from the line lengths thus stored, and sets the maximum line length as the printing width for one line.