A sheet supply apparatus has a sheet storing part that stores sheets, a sheet transport part that feeds the sheets from the sheet storing part and transports the sheets, a sheet separating part that separates the sheets fed from the sheet storing part, a sheet thickness measuring part that measures a thickness of the sheets separated by the sheet separating part, a sheet separating strength varying part that causes the sheet separating part to apply one or more of a plurality of sheet separating strengths to separate the sheets, and a control part that controls the sheet separating strength varying part to cause the sheet separating part to apply, from among the plurality of sheet separating strengths, one or more sheet separating strengths, other than a weakest sheet separating strength, during a time period which begins upon a start of feed of the sheets.