1. An image forming apparatus comprising: a plurality of image bearing members; electrostatic image forming means for forming an electrostatic image on each of said image bearing members; a belt member for carrying a toner image transferred from each of said image bearing member; an electrostatic image transfer member for transferring an electrostatic index image, onto an electrostatic image transfer area located adjacent to an image area of the toner image with respect to a widthwise direction of said belt member, formed on the upstreammost image bearing member with respect to a rotational direction of said belt member; an antenna potential sensor for detecting an induced current, with rotation of said belt member, of the electrostatic index image in the electrostatic image transfer area; control means for controlling superposition of the toner images, formed on said image bearing members and to be transferred onto the image area, through detection of the electrostatic index image in the electrostatic image area by said antenna potential sensor; and setting means for setting an electrical condition, when the electrostatic index image is transferred onto the electrostatic image transfer area during image formation, on the basis of a detection result of the electrostatic index image which is formed during non-image formation, transferred onto the electrostatic image area under an electrical condition different from that during the image formation, and then is detected by said antenna potential sensor.