1. An image processing apparatus comprising: a first resolution conversion unit which executes a resolution conversion processing for image data, which is inputted on a block basis, on the block basis; a conversion unit which converts the image data processed by said first resolution conversion unit on the block basis into a rasterized form; and a second resolution conversion unit which executes a resolution conversion processing on a pixel basis by using the image data in the rasterized form converted by the conversion unit, thereby improving at least one of memory usage and processing time.