Patent ID: 8090277

Claim:
An image forming apparatus comprising: a data obtainer that obtains image data of an RGB system; an image data converter that generates image data of a CMYK system having a C density, an M density, a Y density and a K density by converting the image data of the RGB system; an electrostatic latent image carrier where an electrostatic latent image corresponding to the image data of the CMYK system generated by the image data converter is formed; a developing unit that stirs toners of C, M, Y and K to be supplied to the electrostatic latent image carrier; a driving amount calculator that calculates a driving amount of the developing unit from a predetermined time point in the past to a present time point; a K toner amount calculator of necessary consumption that calculates a K toner amount of necessary consumption corresponding to a K toner amount to be consumed to prevent toner deterioration at the driving amount; a K toner amount calculator of cumulative consumption that calculates a K toner amount of cumulative consumption corresponding to a K toner amount actually consumed at the driving amount; and a K toner amount calculator of consumption insufficiency that calculates a K toner amount of consumption insufficiency obtained by subtracting the K toner amount of the cumulative consumption from the K toner amount of the necessary consumption, wherein in order that black in the image data of the RGB system is expressed by a mixture of four colors C, M, Y and K in the image data of the CMYK system, the image data converter sets the C density, the M density and the Y density to a density obtained by multiplying a density of black in the RGM system by a CMY mixture ratio, and sets the K density to a density obtained by multiplying the density of black by a K mixture ratio, the image data converter sets the K mixture ratio to a K reference mixture ratio when TK<PHK, and the image data converter sets the K mixture ratio to a K high mixture ratio that is higher than the K reference mixture ratio when TK≧PHK, where TK is the K toner amount of the consumption insufficiency and PHK is a K high predetermined value.