Sheet transporting device, and automatic document feeder and image forming apparatus provided with the same

A sheet transporting device comprising:

• a sheet feeding section that feeds plural sheets one by one to a transporting path;
• three or more roller pairs that are arranged at predetermined positions along the transporting path and that nip and transport a sheet;
• a double feed detecting section that detects a length of a double feed portion, which is an overlapped portion, when sheets are fed in such a manner that another sheet (a delaying sheet) is overlapped with one sheet (a preceding sheet);
• a selecting section that selects two of the three or more roller pairs based on the length of the double feed portion;
• and
• a transport control section that can change a transporting speed of at least one of the selected roller pairs,
• wherein the selecting section selects two roller pairs that have a space between which the double feed portion can be positioned, and the transport control section controls the transporting speeds of the selected roller pairs such that the speeds differ from each other to separate the delaying sheet from the preceding sheet,
• the selecting section selects two roller pairs such that one or more roller pair(s) are positioned between the selected two roller pairs, the roller pair(s) positioned between the selected two roller pairs being switched between a transporting state for transporting a sheet and a free state in which the sheet can freely move, and
• the transport control section changes a state of the roller pair(s) positioned between the selected roller pairs into the free state for separating the sheets.