An image data processor for use in an image forming apparatus in which image data of one page are received from outside and bit data based on the image data are supplied to a printing portion in a predetermined quantity each time such that a whole of an image of the one page is printed, the image data processor comprising: an analysis means for sequentially analyzing the image data received from the outside; a data formulating means for sequentially formulating the bit data in accordance with analytic results of the image data by the analysis means; first and second memories for storing the bit data formulated by the data formulating means; the first memory having a memory capacity enabling storage of the predetermined quantity of the bit data; an output means for outputting to the first memory, the bit data stored in the second memory; and a supply means for supplying to the printing portion, the predetermined quantity of the bit data stored in the first memory.