The sheet processing apparatus includes a sheet-discharge fan that blows air downward of a sheet-discharged by an upper sheet-discharge roller, and a control portion configured to control an air velocity of air blown from the sheet-discharge fan according to a basis weight of the sheet. In the case where the basis weight of the sheet is a predetermined value or less, the control portion increases the air velocity of the sheet-discharge fan according to the basis weight of the sheet based on basis weight information from an input portion for inputting the basis weight information of the sheet. The control portion also controls the air velocity of the sheet-discharge fan to be a velocity at which the sheet whose rear edge contacts the sheet-stacking wall does not move in a sheet-discharge direction in the case where the basis weight of the sheet exceeds the predetermined value.