Image forming apparatus

An image forming apparatus comprising:

• a plurality of color image forming stations including a yellow image forming station configured to form a yellow toner image, a magenta image forming station configured to form a magenta toner image, a cyan image forming station configured to form a cyan toner image, and a black image forming station configured to form a black toner image;
• a transparent image forming station configured to form a transparent toner image;
• a transfer station configured to transfer the yellow toner image, the magenta toner image, the cyan toner image, the black toner image and the transparent toner image, which are formed by said color image forming stations and said transparent image forming station, respectively, to a sheet;
• a detector configured to optically detect a density of each of a test yellow toner image, a test magenta toner image, a test cyan toner image, a test black toner image and a test transparent toner image which are formed by said color image forming stations and said transparent image forming station, respectively;
• a controller configured to control each image forming condition of said color image forming stations and said transparent image forming station on the basis of an associated density detected by said detector;
• and
• a counter configured to count a number of sheets on which an image forming process was performed in said apparatus,
• wherein said controller controls said color image forming stations to form the test yellow toner image, the test magenta toner image, the test cyan toner image and the test black toner image when the number of the sheets counted by said counter reaches a first value, and controls said transparent image forming station to form the test transparent toner image when the number of the sheets counted by said counter reaches a second value which is greater than the first value.