PURPOSE: To rapidly and accurately detect a texture boundary by binarizing image data by a required threshold, differentiating the numerical data of the number of picture elements in a texture direction such as a horizontal line and the horizon and defining the peak of a differentiated waveform as a boundary.

CONSTITUTION: The image data of 8 bits or the like outputted from a frame memory 11 are converted into a binary image by a binarizing circuit 14 based upon a threshold set up by a threshold setting circuit 13 correspondingly to the calculated result of a brightness information calculating circuit 12. A picture element number calculating circuit 15 calculates the number of picture elements of the image in the texture direction, and when the numerical level of the number of picture elements exceeds a set level, a picture element number level deciding circuit 16 controls the circuit 13 and the threshold is changed so as to be less than the set level. The numerical data of the number of picture elements of the binary image based upon the threshold in the texture direction are processed by a numeral differentiation circuit 17 and a differential waveform peak corresponding to a texture boundary is detected by a peak detecting circuit 18. Thereby, the texture boundary can be rapidly and accurately detected based upon a settable and clear decision reference without dividing an image and executing large scale calculation.