An image forming apparatus include an image carrier, an intermediate transfer member, a primary transfer member, a power supply unit to form a primary-transfer electric-field between the image carrier and the primary transfer member by applying a primary transfer current to the primary transfer member by performing constant-current control and to detect a change in a primary transfer voltage in a sub-scanning direction that is applied to the primary transfer member from the power supply unit in the sub-scanning direction, a conductive member, a secondary transfer member, a secondary-transfer electric-filed forming member to apply a secondary transfer current to the secondary transfer member or the conductive member, and a control device to control the secondary transfer current applied to the secondary transfer member or the conductive member in accordance with the change in the primary transfer voltage in the sub-scanning direction detected by the power supply unit.