Image forming apparatus

An image forming apparatus comprising:

• an annular transferring belt rotating in a predetermined direction;
• a plurality of photosensitive drums disposed along a rotation direction of the transferring belt;
• a plurality of primary transferring parts transferring images respectively formed on the plurality of photosensitive drums to the transferring belt;
• a transfer voltage applying part applying respective voltages to the plurality of primary transferring parts;
• a resistance value measuring part measuring a subject resistance value of the primary transferring part;
• a cleaning device collecting a remained toner on the transferring belt;
• a pre-brush applying a cleaning charge voltage of the same polarity as the remained toner to the remained toner on the transferring belt at an upstream side from the cleaning device in the rotation direction of the transferring belt;
• a pre-brush current detecting part detecting a pre-brush current flowing through the pre-brush;
• and
• a transfer voltage correcting part, on the basis of a subject resistance value of the primary transferring part measured by the resistance value measuring part, correcting a transfer voltage with respect to the primary transferring part,
• wherein, a case where a change quantity of the pre-brush current detected by the pre-brush current detecting part from the last transfer voltage correction by the transfer voltage correcting part is equal to or more than a predetermined change threshold is defined as a measurement condition,
• if the measurement condition is satisfied, the resistance value measuring part executes resistance value measurement measuring the subject resistance value of the primary transferring part and the transfer voltage correcting part executes the transfer voltage correction correcting the transfer voltage of the primary transferring part.