A multi-color image-forming apparatus having image-forming sections for a plurality of different colors can include an optical sensor having a light-casting section that casts single polarized light and a light-receiving section that receives polarized light different from the cast light. A pattern for detecting toner positions can be used in which toner having high reflectance to a particular light emission wavelength is independently formed. In addition, other patterns in which toner having low reflectance to the particular light emission wavelength are partially formed on a central portion of the pattern of high reflection toner by using the high reflection toner as a foundation. The apparatus can detect each toner pattern by the optical sensor to correct the color shift of each color.