A sheet transport device includes a sheet supporting surface which receives a plurality of sheets set thereon, a separation roller which separates the sheets set on the sheet supporting surface sheet by sheet to transport a separated sheet in a predetermined transport direction, and a first detection unit which is disposed to the supporting surface and detects whether a sheet is set on the sheet supporting surface. The separation roller and the first detection unit are disposed to be separated at least in a sheet width direction orthogonal to the predetermined transport direction, and a distance between the first detection unit and an end edge of the separation roller at a side of the first detection unit is longer than a short side of a non-allowable sheet which is smaller than a minimum sheet size within an allowable transport range set in the sheet transport device.