Image forming apparatus with control for setting transfer bias

An image forming apparatus comprising:

• a photosensitive member charged to a predetermined polarity and configured to bear a toner image;
• an intermediary transfer belt in contact with the photosensitive member at a primary transfer portion and on which the toner image is primary-transferred in the primary transfer portion from the photosensitive member, the primary-transferred toner image being secondary-transferred in a secondary transfer portion;
• a primary transfer member configured to contact an inner peripheral surface of the intermediary transfer belt, and to transfer the toner image onto the intermediary transfer belt from the photosensitive member by a transfer bias being applied;
• an electrode member disposed on an inner peripheral surface side of the intermediary transfer belt, and downstream of a downstream end of the primary transfer portion and upstream of the secondary transfer portion with respect to a rotational direction of the intermediary transfer belt;
• a first applying portion configured to apply a bias of an opposite polarity to the predetermined polarity to the primary transfer member;
• a second applying portion configured to apply a bias of the same polarity as the predetermined polarity to the electrode member;
• a first detecting portion configured to detect a current flowing through or a voltage applied to the primary transfer member;
• a second detecting portion configured to detect a current flowing through or a voltage applied to the electrode member;
• and
• a control portion configured to execute, during non-image formation, a setting operation in which the transfer bias to be applied during image formation is set by causing the first applying portion to apply a test bias to the primary transfer member,
• wherein in executing the setting operation, the control portion sets the transfer bias based on a first detecting result detected by the first detecting portion while the test bias is applied to the primary transfer member and a second detecting result detected by the second detecting portion in the setting operation.