An image forming apparatus includes an image bearing member, an intermediary transfer belt containing an ion conductive agent, a secondary transfer member, and a charging device. With respect to the intermediary transfer belt, when a difference in number of digits (or in log value) between resistivity measured by using a metal probe and resistivity measured by using a sputtering electrode is ΔM, a difference in number of digits (or in log value) between resistivity in a first environment and resistivity in a second environment higher than the first environment in temperature or humidity is ΔE, a length of the secondary transfer portion with respect to the movement direction of the intermediary transfer belt is La, and a distance from the charging portion to the primary transfer portion with respect to the movement direction is Lb, the following relationship is satisfied: ΔM≧ΔE−log(Lb/La).