An image forming apparatus includes an endless belt, an index provided at part of an end edge of the belt, a detecting member, a processing unit, and a storage unit. The detecting member changes an output value thereof based on the passing states of the index and the end edge of the belt in a detection area of the detecting member. The processing unit processes the output value and determines whether the index is present in the detection area by comparing the present output value with a threshold value. The storage unit stores the processed output value. The processing unit acquires a reference value based on the stored output value. The processing unit, when determining whether the index is present in the detection area, sets as the threshold value a value obtained by changing the most recently acquired reference value by a predetermined value.