An image forming apparatus connectable to a sheet post processing device, the sheet post processing device capable of changing a conveying speed when the sheet post processing device receives a sheet, the apparatus comprising: discharging means arranged to discharge a sheet in any of a reverse discharge mode in which a sheet is discharged to the sheet post processing device with front and back surfaces of the sheet after image formation reversed, and a non-reverse discharge mode in which a sheet is discharged to the sheet post processing device without front and back surfaces of the sheet after image formation reversed, the discharging means arranged to discharge the sheet to the sheet post processing device at a first speed or a second speed which is lower than the first speed; and control means configured to control the discharging means to discharge the sheet at the first speed when the sheet is to be discharged in the reverse discharge mode, and to discharge the sheet at the second speed when the sheet is to be discharged in the non-reverse discharge mode, characterized in that, when a previous sheet is discharged at one of the first speed and the second speed, the control means is configured to control the discharging means to discharge a next sheet discharged immediately after the previous sheet at the same speed as the previous sheet, regardless of which mode the next sheet is to be discharged in; wherein, when a previous sheet is discharged at the first speed and the next sheet is to be discharged in the non-reverse discharge mode, the control means is configured to control the discharging means to discharge the next sheet at the first speed only in the case that the next sheet has a length equal to or less than a predetermined length in a conveying direction. discharging means arranged to discharge a sheet in any of a reverse discharge mode in which a sheet is discharged to the sheet post processing device with front and back surfaces of the sheet after image formation reversed, and a non-reverse discharge mode in which a sheet is discharged to the sheet post processing device without front and back surfaces of the sheet after image formation reversed, the discharging means arranged to discharge the sheet to the sheet post processing device at a first speed or a second speed which is lower than the first speed; and control means configured to control the discharging means to discharge the sheet at the first speed when the sheet is to be discharged in the reverse discharge mode, and to discharge the sheet at the second speed when the sheet is to be discharged in the non-reverse discharge mode, characterized in that, when a previous sheet is discharged at one of the first speed and the second speed, the control means is configured to control the discharging means to discharge a next sheet discharged immediately after the previous sheet at the same speed as the previous sheet, regardless of which mode the next sheet is to be discharged in; wherein, when a previous sheet is discharged at the first speed and the next sheet is to be discharged in the non-reverse discharge mode, the control means is configured to control the discharging means to discharge the next sheet at the first speed only in the case that the next sheet has a length equal to or less than a predetermined length in a conveying direction.