A sheet transport device is provided with a first cassette (1) for containing therein standardized copy sheets having a small thickness and various sizes, a first transport path (50) along which the standardized copy sheets dispensed from the first cassette (1) are transported, a first sheet separator (5) disposed in a specified position along the first transport path (50) for separating the standardized copy sheets from one another, the first sheet separator (5) including a first forward roller (18) which is rotated in a direction along transport of sheet, a first retard roller (21) which is disposed in parallel to the first forward roller and rotatable in a direction against the transport of sheet, a drive mechanism (17) for transmitting the driving force to the first retard roller (18), the first forward roller (18) and first retard roller (21) being held in contact with each other, a second cassette (2) for containing therein either the standardized copy sheets or special sheets having a thickness larger than that of the standardized copy sheet and various sizes, a second transport path (60) along which the sheets dispensed from the second cassette means (2) are transported, and a second sheet separator (6) disposed in a specified position along the second transport path (60) for separating the special sheets from one another, the second sheet separator (6) including a second forward roller (32) which is rotated in a direction along the transport of sheet and a second retard roller (33) which is rotatable in a direction against the transport of sheet and spaced away from the second forward roller (32) by a distance (S) corresponding to the thickness of the first sheet. Accordingly, even when the thick special sheets such as envelops are fed either in line or side by side, multiple feeding of the special sheets can be effectively prevented.