ROTATION-INFORMATION DETECTING DEVICE, ROTATION CONTROL DEVICE, AND IMAGE FORMING APPARATUS

A rotation-information detecting device including: a rotatable rotary member having plural detection target portions arranged at predetermined intervals over an entire circumference of the rotary member; two detectors that are fixedly arranged at two positions of the rotary member in a rotating direction, and can detect the detection target portions; and a computation unit that computes rotation information of the rotary member based on detection information, wherein the two detectors are arranged along the circumferential direction of the rotary member at an interval of an angle of π/N, and wherein the computation unit includes an offset calculating section that offsets an eccentric error of the rotary member based on outputs of the two detectors at a current time point and n outputs at time points back from the current time point by phases of (nπ)/N, to calculate a true rotational error.