Image forming apparatus with detection part that detects color concentration detection pattern

An image forming apparatus in which an image forming part that forms a developer image is located close to an endless intermediate transfer belt, configured to transfer the developer image that is formed by the image forming part to the intermediate transfer belt at a first transfer position and to carry the developer image, then to transfer the developer image on the intermediate transfer belt to a sheet at a second transfer position, the image forming apparatus comprising:

• a detection part located between the first transfer position and the second transfer position and configured to detect a concentration of the developer image on the intermediate transfer belt;
• and
• a controller configured
• to start a transfer of a concentration detection pattern that is a developer image for concentration detection to the intermediate transfer belt during a period from when the developer image for print that is the developer image based on print data input from an external part is transferred to the intermediate transfer belt to when the developer image for print is transferred to the sheet, and then
• to control the detection part to read the concentration detection pattern, wherein
• the image forming part is configured from a plurality of color image forming parts that correspond to respective colors,
• the controller comprises a color judgment part that judges whether or not a predetermined condition for each color developer is satisfied for forming the concentration detection pattern, and
• the controller causes the color image forming part to generate the concentration detection pattern using only the color developer for which the color judgment part judges that the predetermined condition has been satisfied.