PURPOSE: To enable synthesizing of color CRT display data into monochromatic printer output data without any load brought to bear on a computer by providing a control means to red red, green and blue from a memory simultaneously and a data synthesizing circuit which makes a conflux of red, green and blue to obtain a logic sum of each data.

CONSTITUTION: MPU 1 instructs a latching circuit 7 so that all RAM's can be read. The latching circuit 7 outputs a latching signal to a timing control circuit 8 so that the latter outputs a control signal which enables reading of all RAM's 21W23. MPU 1 reads R, G, B display data from RAM's 21W23 simultaneously. The R, G, B display data is synthesized by a logic sum of each data obtained by its conflux through diodes 31R, 31G and 31B of a data synthesizing circuit 3 and consequently, the R, G, B display data thus synthesized become printer output data. This data is once stored in a memory of MPU 1 through a bidirectional gate 4, and output to a printer 10 through a printer interface 9. The printer 1 prints out in monochromatic mode.