PURPOSE: To accurately display a thickness line having a designated three-dimensional thickness amount in two orthogonal two-dimensional line drawings on which a three- dimensional object being a curved surface is projected.

CONSTITUTION: A tangential angle generating means 1 generates two tangential angles formed by the tangential line of a curve at a designated point, which is a single point in a three-dimensional space and belongs to some respective curves in the two-dimensional drawings and a perpendicular line in parallel to an axis shared by the two-dimensional line drawings, and a projective thickness amount generating means 2 generates the projective thickness amount being two two-dimensional thickness amounts projected on two planes, to which the two-dimensional line drawings belong, from the two tangential angles and the designated three-dimensional thickness amount, and a thickness amount synthetic display means 3 displays a thickness point by synthetizing the two projective thickness amounts on the perpendicular line from the two projective thickness amounts and the two tangential angles. A thickness line display means 4 displays the thickness line with the displayed thickness point group interpolated by repeating a tangential line angle generating means 1, the projective thickness amount generating means 2 and the thickness amount synthetic display means 3.