1. An image forming apparatus comprising: a movable first image bearing member; a first latent image forming unit configured to form an electrostatic latent image, including a first latent image and a first latent index pattern which are based on an image signal, on said first image bearing member; a movable belt member configured to contact said first image bearing member; a first toner image transfer member configured to transfer a first toner image, obtained by depositing a toner on the first latent image, onto said belt member; a latent pattern transfer member configured to be disposed at a position corresponding to the first latent index pattern with respect to a widthwise direction of said belt member and to be supplied with a latent pattern transfer voltage, to transfer the first latent index pattern from said first image bearing member onto said belt member; a movable second image bearing member configured to be disposed at a position downstream of said first image bearing member with respect to a moving direction of said belt member and to contact said belt member; a second latent image forming unit configured to form an electrostatic latent image, including a second latent image and a second latent index pattern which are based on an image signal, on said second image bearing member; a second toner image transfer member configured to transfer a second toner image, obtained by depositing the toner on the second latent image, onto said belt member; a first latent pattern detector configured to be disposed opposed to said belt member at the position corresponding to the first latent index pattern with respect to the widthwise direction and at a position downstream of said latent pattern transfer member with respect to a moving direction of said belt member, and configured to detect a transferred first latent index pattern transferred by said latent pattern transfer member from said first image bearing member to said belt member; a second latent pattern detector configured to be disposed opposed to said second image bearing member at a position corresponding to the second latent index pattern with respect to the widthwise direction and at a position downstream of said second latent image forming unit, and configured to detect the second latent index pattern; an adjusting portion configured to adjust a relative position between the first latent index pattern and the second latent index pattern on said belt member with respect to the moving direction of said belt member, on the basis of detection results of said first latent pattern detector and said second latent pattern detector; and a setting portion configured to set the latent pattern transfer voltage, during image formation, on the basis of a detection result of said first latent pattern detector on condition that a plurality of test voltages are applied to said latent pattern transfer member during non-image formation.