PURPOSE: To easily alter a function into a function that a user intends by displaying a display area consisting of a section X and a section Y and the curve included in it, setting plural sections in the X direction of the display area, and inputting Y values for respective set points and generating the secondary function from X and Y.

CONSTITUTION: A memory 3 is stored with ink thickness correction curves of the respective colors used by a color BJ printer driver together with a control program and data used for the arithmetic of a CPU 2. The control program includes a function curve alteration program for altering respective ink thickness correction curves. The CPU 2 is supplied with operation indications and data from an input device 4 and has a keyboard including plural operation keys, a mouse, etc. The arithmetic result of the CPU 2 is displayed on a display 1 and also printed out on recording paper by a printer 5.