PURPOSE: To execute window processing by utilizing neighbourhood information stored in a neighbourhood information memory for executing thinning processing.

CONSTITUTION: At the time of starting the thinning processing, the information of a picture element neighboring upon a remarked picture element is read out from the neighbourhood information memory 2 and inputted to a deciding processing means 4 and the binary data of the remarked picture element are inputted from a binary data of the remarked picture element are inputted from a binary data of the remarked picture element are inputted from a binary image memory 1 to the means 4. When the inputted heighbourhood information coincides with a deletion pattern stored in the means itself 4 and the binary data of the remarked picture element is '1', the means 4 decides the deletion of the binary data of the remarked picture element and rewrites the data of the remarked picture element concerned stored in the memory 1 to '0'. After the end of processing of the means 4, a neighbourhood information forming means 5 forms new neighbourhood information based upon the undated binary data and writes the formed information in the memory 2.