M×N attribute data for M×N pixels, where M and N are integer multiple of “L” are input. Data expressed by L bits are encoded, A first encoding control step supplies the M×N attribute data to be encoded in an input order, causing encoding to be executed. A second encoding control step converts the M×N attribute data input into bit planes of bit 0 to bit L−1, extracts L-bit data from one converted bit plane, and supplies the extracted L-bit data for each bit plane, causing encoding to be executed. An amount of encoded data generated by the encoding under the control of the first encoding control step is compared with an amount of encoded data generated in the encoding under the control of the second encoding control step, and encoded data with a smaller amount are selected. The selected encoded data and identification information representing the selected data are then output.