A control portion obtains a first difference in a movement amount between paper passing period and paper non-passage period of a transfer belt, and determines whether or not a first absolute value thereof is a first threshold value or more. In a case where the first absolute value is the first threshold value or more, a transfer belt unit positional adjustment motor is driven to adjust a first angle with respect to a first direction of a transfer belt unit with respect to a photoreceptor drum. In a case where the first absolute value is less than the first threshold value, a second absolute value of a second difference between a movement amount and a designed value set in advance of the transfer belt is obtained, and whether or not the second absolute value is a second threshold value or more is determined. When the second absolute value is less than the second threshold value, the processing is finished, and when the second absolute value is the second threshold value or more, a second angle with respect to a second direction orthogonal to the first direction of the driving roller with respect to the photoreceptor drum is adjusted so as to be less than the second threshold value by a driving roller positional adjustment motor.