1. A color image forming apparatus for converting tonality of input image data using an image conversion table and for forming an image on a transfer sheet based on the converted input image data, comprising: a first optical detection unit configured to detect light reflecting characteristics of an unfixed toner image formed on an image carrier or a transfer sheet carrier; a second optical detection unit configured to detect light reflecting characteristics of a fixed test toner image which is formed based on test pattern data and transferred from the image carrier or the transfer material carrier onto a transfer sheet; a control unit configured to form the fixed test toner image on the transfer sheet based on the test pattern data and to cause the second optical detection unit to detect the fixed test toner image on the transfer sheet and to modify the image conversion table, in accordance with a detection result of detecting the fixed test toner image by the second optical detection unit, wherein the image conversion table converts tonality of the input image data so that correlation between the tonality of the input image data and light reflecting characteristics of a toner image on a transfer sheet based on the input image data becomes a prescribed correlation; and a determination unit configured to determine to cause the control unit to form the fixed test toner image on the transfer sheet and to cause the second optical detection unit to detect the light reflecting characteristics of the fixed test toner image on the transfer sheet, in a case where a detection result provided by the first optical detection unit is different from a target value by not less than a predetermined amount.