To transmit reliable image data at a low price.

The image forming apparatus includes a controller part 1 with FIFO 19 to 22 for holding image data, a serial data transfer part 2 for transmitting the image data of each color through time sharing for each color from the controller part 1, and a printer engine part 3 with FIFO 33 to 36 for holding the transmitted image data temporarily for each color. FIFO status detection part 49 detects the empty situation of FIFO 33 to 36 for each color and decides the color with the largest empty, and then notifies the color to the controller part 1 as a data request. The controller part 1 chooses the color of the image data transmitted according to the notified data request, and the image data of the chosen color are transmitted to the printer engine part 3 by the serial data transfer part 2.