PURPOSE: To reduce calculation related to the hidden-line processing of a three- dimensional curve surface and the load of a storage area.

CONSTITUTION: In a plotting processing method on the screen of the three- dimensional curve surface showing a function (z) consisting of a variable (x) and a variable (y), a step 101 where a first ridge line is sequentially plotted by the respective values of the variable (y) at the prescribed value of the variable (x) on an x-coordinate becoming a glance from the variable (x) farthest from the visual point, a step 102 where a second ridge line is plotted by the value of the next variable (x) in the respective values of the variable (y) of the first ridge line, a step 103 where a closed curve line is generated by connecting the both ends of the first ridge line at the next variable (x) and the orthogonal projection of the first ridge line on an xy-plane whenever the plotting of the second ridge line completes, a step 104 where the internal side of the closed curve line is painted out by the background of the screen and a step 105 where the first ridge line at the next variable (x) is plotted after the paint-out are included.