A matrix unit receives a luminance signal L* for a uniform color space and two color difference signals a* and b*. Then, when a control signal has given instructions for a mono-color editing operation, the matrix means outputs only the luminance signal L* out of the picture image data fed into it, and, when a control signal is given otherwise, the matrix means generates and outputs toner color signals for the colors Y, M, C, and K. A multiplying unit multiplies with a prescribed coefficient the picture image data which are output from the matrix unit, and the multiplying unit multiplies the picture image data with a coefficient for permitting the toner signal output from the matrix unit to pass through in case the control signal has not given any instructions for any mono-color editing operation, and multiplies the input luminance signal L* with four coefficients corresponding to the color as instructed by the control signal and generate four toner color signals for Y, M, C, and K. The coefficients have the data form representing the coverage of the toner colors, Y, M, C, and K.