To provide an image forming device for switching back a paper sheet by paper discharge roller pairs capable of preventing jamming caused by the skew of the paper sheet without considerably increasing the number of components and the manufacturing cost.

A plurality of branch claws 61a to 61h which are oscillatingly mounted on a shaft 60 provided vertically with respect to the conveying direction is used for a branching means. The oscillation of the branch claws 61a, 61h provided at least on both sides of the shaft 60 is detected by sensors Sa, Sh. The skew amount of a paper sheet P is detected from the detection time difference of the oscillation of the branch claws 61a, 61h provided on both sides of the shaft 60 when the paper sheet P passes through the branch claw 61 is detected. When the detection time difference is not less than the predetermined value, it is determined that the skew amount of the paper sheet is large, and the paper sheet is discharged without any switch-back.

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