An image processing apparatus and its method which easily change the definition of pixel attribute in an image processing unit even if the image processing unit has been realized by hardware such as a semiconductor integrated circuit. The image processing apparatus inputs image data and first pixel information indicating information on a pixel of the image data. A first selector 101 selects predetermined bits from the first pixel information and generates second pixel information. A second selector 103 selects one of the plural third pixel information stored in a register file 102 based on the second pixel information and outputs the selected third pixel information as fourth pixel information. A decoder 104 determines a pixel attribute based on the fourth pixel information. Then, image processing is performed on the image data based on the determined pixel attribute. At this time, the selection bits by the first selector and the data stored in the register file 102 are variable.