An image forming apparatus comprising a conveyer-member control device that controls a conveying speed of a conveyer member and an image forming unit (6A, 6B, 6C, 6D) that forms an image on the conveyer member, the conveyer-member control device comprising: a detecting unit that detects a conveying speed of a conveyer member; a driving unit that drives the conveyer member; a first control unit that performs a first feedback control of the conveying speed based on detected conveying speed and a predetermined target conveying speed; a second control unit that detects a rotation speed of the driving unit and performs a second feedback control of the rotation speed based on detected rotation speed and a predetermined target rotation speed; a determining unit (340, 1240) that determines whether the detecting unit is anomalous based on the detected conveying speed while the first feedback control is performed; and a switching unit (330, 630, 830, 835) that switches, when determining unit (340, 1240) determines that the detecting unit is anomalous, from the first feedback control to the second feedback control; characterized by a counting unit that counts a number of times that the determining unit determines that the detecting unit is anomalous, wherein the switching unit (330, 630, 830, 835) switches from the first feedback control to the second feedback control when the number of times exceeds a predetermined threshold; a color-printing determining unit that determines whether the image is a color image or a monochrome image, wherein the detecting unit further detects a pulse signal corresponding to the conveying speed, and the determining unit determines that the detecting unit is anomalous in either one of a case where the image is a color image and a pulse period of the pulse signal exceeds a first range set for a plurality of colors, and a case where the image is a monochrome image and the pulse period exceeds a second range set for a single monochrome color. a detecting unit that detects a conveying speed of a conveyer member; a driving unit that drives the conveyer member; a first control unit that performs a first feedback control of the conveying speed based on detected conveying speed and a predetermined target conveying speed; a second control unit that detects a rotation speed of the driving unit and performs a second feedback control of the rotation speed based on detected rotation speed and a predetermined target rotation speed; a determining unit (340, 1240) that determines whether the detecting unit is anomalous based on the detected conveying speed while the first feedback control is performed; and a switching unit (330, 630, 830, 835) that switches, when determining unit (340, 1240) determines that the detecting unit is anomalous, from the first feedback control to the second feedback control; characterized by a counting unit that counts a number of times that the determining unit determines that the detecting unit is anomalous, wherein the switching unit (330, 630, 830, 835) switches from the first feedback control to the second feedback control when the number of times exceeds a predetermined threshold; a color-printing determining unit that determines whether the image is a color image or a monochrome image, wherein the detecting unit further detects a pulse signal corresponding to the conveying speed, and the determining unit determines that the detecting unit is anomalous in either one of a case where the image is a color image and a pulse period of the pulse signal exceeds a first range set for a plurality of colors, and a case where the image is a monochrome image and the pulse period exceeds a second range set for a single monochrome color. a case where the image is a color image and a pulse period of the pulse signal exceeds a first range set for a plurality of colors, and a case where the image is a monochrome image and the pulse period exceeds a second range set for a single monochrome color.