PURPOSE: To attain substitute prints without stopping image processing even when a desired development agent is not available by converting print data of a development agent not available into print data of other development agent available and printing out data by the other development agent when at least one color development agent of plural colors is not available.

CONSTITUTION: A print processing unit 12 provides an output of a print request signal PAGE to a printer controller 11. When print is enable, the controller 11 provides an output of a reply signal PAGAK to the processing unit 12 to inform that the print data are receptible. After the controller 11 outputs a vertical synchronizing signal VSYNC and a horizontal synchronizing signal HSYNC to each plane (print plane of each color), the processing unit 12 provides an output of print data VIDEO. When no yellow development agent is available, it is informed from the controller 11 to the processing unit 12 by serial communication 21. The processing unit 12 converts print data of the development agent not available into print data of other available development agent and printing is made by the other development agent.