Image data supplied to an image forming apparatus are converted into image data linearly correlated with the image density, and the image ratio is calculated by dividing the number of pixels in which the toner is deposited during image formation based on the thus-converted image, with a value obtained by multiplying the total pixel number corresponding to the size of the print medium with the number of gradation levels per pixel. Then the toner amount consumed in the image formation based on the entered image data is determined by multiplying the image ratio, the size of the print medium to be used and the toner amount consumed per unit area in a solid image formation.