In order to reduce the capacity of a line memory when converting image data that are input in a raster scan order into pixel block data such as 8×8 pixels for JPEG compression and the like, data on the first 0th to 7th lines is written in an 8H line memory in a raster scan order. Next, while performing block reading, the writing of the next 8th to 15th lines is performed so as to follow the part that has been read. The block reading of the 8th to 15th lines is performed in an order from the area where the data on the 0th line was first stored to the area where the data on the 1st, 2nd . . . lines was stored. For this reason, the writing of the 16th to 23rd line that is performed simultaneously with the block reading of the 8th to 15th lines can be performed in the same manner as the writing of the first 0th to 7th lines. Therefore, block conversion can be performed using an 8H line memory capacity corresponding to 8 lines, whereas that corresponding to 16 lines has been conventionally required.