Image forming apparatus and image forming method for preventing retransfer of toner images from a conveying member to sequentially ligned image carriers

A tandem image forming apparatus and method including a conveying or intermediate member (23) for transporting a toner image in a sub-scanning direction, a plurality of image carriers (19) for contacting the conveying or intermediate member at equal intervals (T1) between the contacting portions, a contact force adjusting mechanism (28) configured to freely adjust the contact force of the conveying or intermediate member to the plurality of image carriers between a predetermined contact force and a decreased contact force, a plurality of toner image forming devices to form toner images of a predetermined pattern on, respectively, the plurality of image carriers, the length of the pattern in the sub-scanning direction being set shorter than the length (T1) of the equal intervals between the contacting portions, a plurality of transferring devices (29) for respectively transferring the toner images on the plurality of image carriers (19) onto the conveying or intermediate member (23), and a contact force changing device (31) configured to change the predetermined contact force by the contact force adjusting mechanism (28) to the decreased contact force before respective tips of the transferred toner images on the conveying member (23) pass subsequent contacting portions, so that respective parts of the toner images on the conveying or intermediate member (23) are not transferred back onto subsequent image carriers of the plurality of image carriers (19).