Color image forming apparatus capable of effectively matching registration between elementary color images

An image forming apparatus, comprising:

• a plurality of image forming units including nth and (n+1)th image forming units numbered in an order of a predetermined image forming sequence, in which n is a positive integer, said image forming units are configured to write latent images on rotating image carrying members at write positions and to develop the latent images;
• an intermediate transfer member configured to rotate to pass respective transfer positions at which the developed images are transferred to the intermediate transfer member from the respective image carrying members of the plurality of image forming units;
• a plurality of marks including nth and (n+1)th marks placed along a rotation direction of the intermediate transfer member,
• wherein an interval between the nth and (n+1)th marks is substantially equal to or smaller than a distance between a write position of the nth image forming unit to a primary transfer position of the (n+1)th image forming unit and a distance between a write position of the (n+1)th image forming unit to the primary transfer position of the (n+1)th image forming unit;
• a mark detecting mechanism configured to detect the marks at a fixed position;
• a counter configured to reset and restart counting time upon a detection of each of the marks by the mark detecting mechanism;
• a count value memory unit configured to store a count value Cn counted during a time period from a detection of the nth mark until a start of writing on the image carrying member in the nth image forming unit, and a count value C(n+1) counted during a time period from detection of the (n+1)th mark until a start of writing on the image carrying member in the (n+1)th image forming unit;
• a use frequency memory unit configured to store a number of printed sheets;
• a delay amount memory unit configured to store data representing a relationship between the number of printed sheets and an amount of displacement of a position at which a full-color toner image is formed on the intermediate transfer member;
• and
• a controller configured to reset the counter to start counting time upon detection of the nth and (n+1)th marks, respectively, by the mark detecting mechanism and to instruct the nth image forming unit and the (n+1)th image forming unit to start writing on the image carrying member when the counter reaches the count value Cn, and C(n+1), the controller being configured to change the transfer positions on the intermediate transfer member based on the delay amount output from the delay amount memory unit, as a use frequency of the intermediate transfer member increases.