An abnormality detecting device for a rotation body includes: a rotation body rotating while coming in contact with a sheet transported at a predetermined speed; an output unit outputting pulses of a number in proportion to a rotation amount of the rotation body; an acquisition unit acquiring periodic information associating a position of the rotation body during a single rotation and a period of each of the pulses corresponding to the position with each other, the periodic information being acquired based on the plural pulses outputted from the output unit along with the rotation of the rotation body at the predetermined speed; a memory storing the periodic information as reference periodic information; and an abnormality detector detecting an abnormality occurred in at least one of the rotation body and the output unit based on the reference periodic information and new periodic information acquired after the reference periodic information is acquired.