Image forming apparatus

An image forming apparatus comprising:

• a plurality of image carrying members;
• an intermediate transfer member to which a toner image formed on a surface of the plurality of image carrying members is primarily transferred;
• a plurality of primary transfer members disposed to face the plurality of image carrying members across the intermediate transfer member and configured to primarily transfer the toner image from the plurality of image carrying members to the intermediate transfer member;
• a characteristic measuring portion configured to measure current-voltage characteristics of each of the plurality of primary transfer members before a toner image for a first surface of a sheet is primarily transferred;
• a characteristic information storage portion configured to store characteristic information that indicates the current-voltage characteristics measured by the characteristic measuring portion;
• a first transfer voltage determining portion configured to determine, based on a predetermined first target current value and the characteristic information, a first transfer voltage that is applied to the plurality of primary transfer members when the toner image for the first surface of the sheet is primarily transferred;
• a second transfer voltage determining portion configured to determine, based on a predetermined second target current value and the characteristic information that was used when the first transfer voltage was determined, a second transfer voltage that is applied to the plurality of primary transfer members when a toner image for a second surface of the sheet is primarily transferred, the second target current value being smaller than the first target current value;
• a voltage control portion configured to apply the first transfer voltage to the plurality of primary transfer members by a constant-voltage method when the toner image for the first surface of the sheet is primarily transferred, and apply the second transfer voltage to the plurality of primary transfer members by the constant-voltage method when the toner image for the second surface of the sheet is primarily transferred;
• a plurality of power sources configured to apply voltages respectively and individually to the plurality of primary transfer members;
• and
• a total current detecting portion configured to detect a total current by totaling output currents of the plurality of power sources, wherein
• the characteristic measuring portion measures the current-voltage characteristics of each of the plurality of primary transfer members in sequence based on the total current detected by the total current detecting portion when output voltages of the plurality of power sources are changed in a predetermined pattern.