A density counting unit of an image forming apparatus for forming a monochromatic image or a color image represented by a plurality of color components on a photosensitive member includes: first totaling means for totaling pixel values of monochromatic component of each pixel of an image when a monochromatic image is formed; second totaling means for totaling pixel values of monochromatic component of each pixel of an image when a monochromatic image is formed, and totaling pixel values of predetermined color component when a color image is formed; and third totaling means for totaling pixel values of color components other than the predetermined color component when a color image is formed. When a monochromatic image is formed, the first totaling means and the second totaling means total the pixel values of the odd number lines and the even number lines, respectively.