An image forming apparatus including an image forming unit; a color conversion unit; a storage unit which stores a mixed-color pattern including a gradation of tones of a mixed color and a reference value of each of the tones; and a control unit which forms an image of the mixed-color pattern on the sheet with the image forming unit, compares color values representing color shades for each of the tones between an observed value of the image of the mixed-color pattern formed on the sheet and the reference value of the mixed-color pattern stored in the storage unit, and determines that the multi-dimensional LUT needs to be updated and updates the multi-dimensional LUT if a difference of the color values is determined to be large on the basis of a threshold value.