PURPOSE: To minimize the deviation of driving of each part to reduce the generation of image blur and image deviation, by equalizing moments of inertia in input shafts of two driving units, at least, out of respective driving units of a movable part concerned in image formation.

CONSTITUTION: A photosensitive drum driving unit 15, an original scanning optical system driving unit 17, and a carrying part and fixer driving unit 19 are driven through belts by a motor 13. In respect to driving units 15 and 17, an inertia load is attached to one driving unit having a smaller moment of inertia due to parts required for mechanical constitution so that the value of the moment of inertia of this driving unit is equal to that of the other driving unit having a larger moment of inertia, and elastic forces are given to idlers 21 so that tensions of belts for them are equal to each other, and thus, these driving units are regarded as the same load of the motor. As the result, even if the variance of the rotation speed of the motor occurs, respective unit input shafts take the same action.