To provide a rotor driving device, keeping the rotating speed of a rotor constant by preventing the occurrence of positional shifting due to slippage or the like and detecting a high-accuracy rotational change corresponding to the rotating position of the rotor at low cost, and an image forming apparatus including the rotor driving device, which may form an image of high quality at high speed.

This rotor driving device includes: a rotor driving shaft 2 for driving to rotate the rotor 1; a gear accelerating means 3 formed of a gear train rotated by the rotor driving shaft 2; and a rotor rotational change detecting means 4 provided on the driven gear 3b side having the integer multiples of rotational frequency of a driving gear 3a in the gear accelerating means 3 to detect a rotational change of the rotor 1.