To directly engrave even a line of fine breadth and depth by converting the picture element value of a bit map to depth information, and calculating a three-dimensional coordinate group continuous from a start point to an end point in the line.
A line drawing pattern expressed by means of a free curve and a bit map image expressed by means of variable density are drawn and superposed on each other, thereby finding the values of bit map picture elements 6a and 6b near plane coordinates (X-Y coordinates) on a center line. The values of the bit map picture elements 6a and 6b so obtained are, then, converted to a depthwise coordinate (X-coordinate) at an engraving process. At the same time, a continuos three-dimensional coordinate group (X-Y-Z coordinate group) is calculated over an area from a start point to an end point in each line. The coordinate group so obtained is used as numerical information for the travel position of a bite edge corresponding to a chisel for directly engraving a recessed groove mechanically, and line depth is continuously changed, so as to correspond to the change of the bit map picture element values.