An image forming apparatus includes an image carrier, an intermediate transfer member, a primary transfer member, and a power supply unit to form a primary-transfer electric-field between the image carrier and the primary transfer member by performing constant-current control. The primary transfer member also detects 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. There is a conductive member, a secondary transfer member, and a secondary-transfer electric-field forming member to apply a secondary transfer current to the secondary transfer member or the conductive member. A control device controls 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.