PURPOSE: To perform effective smoothing processing and to extract useful information on a pattern contour stably, by eliminating discontinuity of the direction code string of the pattern contour.

CONSTITUTION: A direction code found by quantizing the direction of contour line tracing at every contour point is inputted to a latch circuit 1. This direction code latched by the circuit 1 is sent to a latch circuit 2 at next timing and stored therein. Therefore, the direction code of some contour picture element is stored in the circuit 1 and the direction code of a contour picture element which is one before is stored in the circuit 2. An ROM3 generating a function F inputs the direction codes stored in the circuits 1 and 2 as address data, and outputs three kinds of function F of +1, 0, and -1 according to the combinations of those direction codes. The function F is supplied through an adder 4 to a latch circuit 5 and accumulated successively. Then, the output data of the adder 4 is outputted to the circuit 1 as additional equivalent conversion control information added to the stored direction codes.