PURPOSE: To detect an edge part of an image drawn on an original in a two-di mensional way by producing an visible outline image having the prescribed picture element width out of a boundary point between an image part and a non-image part detected by a main scanning direction edge detecting means and a secondary scanning direction edge detection means.

CONSTITUTION: A main scanning direction edge detecting means 3 detects a boundary point between an image part I and a non-image part NI on each scanning line L based on the read image information. At the same time, a secondary scanning direction edge detecting means 4 detects a boundary point between the parts I and NI at each picture element position in the moving direction S of the line L based on the read image information. Then a visible outline generating means 5 produces visible outline pictures having the pre scribed picture element width out of the boundary points detected by both means 3 and 4. In such a constitution, the edge part of an image drawn on an original can be detected in a two-dimensional way.