An image processing apparatus which comprises a RAM acting as a shared memory for storing image data, and an image processing board, connected to the RAM through a PCI bus acting as a general-purpose bus, for performing raster conversion to the image data and performing a necessary image process to the converted data by using a line memory is provided. In this apparatus, in a case where the image processing board can not input or output the data from or to the PCI bus, it is controlled not to generate a sync signal used for a process in the image processing board, whereby a stable operation is possible in an image forming apparatus and an image processing method which include the image processing apparatus.