In an image forming device including a first rotary member arranged to drive an intermediate transfer belt, a first motor arranged to rotate the first rotary member, and a second rotary member arranged to transfer a toner image formed on the intermediate transfer belt to a recording sheet, a torque command value detecting unit detects a torque command value to the first motor. A setting unit sets up a lower limit of the torque command value. A judgment unit determines whether a detected torque command value exceeds the lower limit. A control unit controls driving of the second rotary member when the detected torque command value exceeds the lower limits so that a difference between a surface speed of the intermediate transfer belt and a peripheral speed of the second rotary member falls within a predetermined range.