An image forming apparatus 1 includes four developing devices 43, four primary transfer rollers 51, an intermediate transfer belt 54, voltage applicators 61, a current detector 62 and a voltage controller 911. The voltage applicators 61 each apply a voltage to a corresponding one of the four primary transfer rollers 51. The current detector 62 detects a first total current value JS of currents flowing through the four primary transfer rollers 51. The voltage applicators 61 apply a detection voltage VS to a primary transfer roller 51y among the four primary transfer rollers 51 while applying a voltage having the same polarity as the detection voltage VS to all the other primary transfer rollers 51c, 51m, and 51k. The detection voltage VS is applied to detect a resistance value Ry of the primary transfer roller 51y. The detection voltage VS has a predetermined voltage value.