An image forming apparatus includes a primary transfer bias adjuster to adjust, in a second mode, a primary transfer bias by a correction amount in accordance with a degree of deterioration of a developing agent detected by a first developing agent condition detector so as to reduce a primary transfer current of the primary transfer bias upon transferring a toner image from a first image bearing member onto an intermediate transfer member, and to adjust, in a first mode, the primary transfer bias by a correction amount less than the correction amount in the second mode or not to adjust the primary transfer bias in accordance with the degree of deterioration of the developing agent detected by the first developing agent condition detector upon primarily transferring the toner image from the first image bearing member onto the intermediate transfer member.