Image forming apparatus

An image forming apparatus comprising:

• an image bearing member for bearing a toner image;
• a rotationally movable intermediary transfer belt onto which the toner image is to be primary-transferred from said image bearing member at a primary transfer portion where said intermediary transfer belt contacts said image bearing member, wherein said intermediary transfer belt has electroconductivity by containing an ion conductive agent;
• a secondary transfer member for secondary-transferring the toner image from said intermediary transfer belt onto a transfer material at a secondary transfer portion where said intermediary transfer belt contacts the transfer material;
• and
• a charging device for electrically charging said intermediary transfer belt or a toner on said intermediary transfer belt at a charging portion positioned downstream of the secondary transfer portion and upstream of the primary transfer portion with respect to a movement direction of said intermediary transfer member,
• wherein when a difference in number of digits between resistivity of said intermediary transfer belt measured by using a metal probe and resistivity of said intermediary transfer belt measured by using a sputtering electrode is ΔM, a difference in number of digits between resistivity of said intermediary transfer belt in a first environment and resistivity of said intermediary transfer belt 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 said 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).