An image forming apparatus comprising: a post-processing device configured to perform punching processing on a sheet on which an image is formed and stapling processing on a sheet bundle consisting of a plurality of the punched sheets; a punching time measuring unit configured to measure a punching processing time to perform the punching processing; a stapling time estimating unit configured to calculate an estimated stapling processing time to perform stapling processing by using the punching processing time measured by the punching time measuring unit;and a sheet-bundle interval time setting unit configured to set, by using the estimated stapling processing time, a sheet-bundle interval time, which is a time from when the last sheet forming a sheet bundle subjected to the punching processing is fed until when the first sheet forming a subsequent sheet bundle subjected to the punching processing is fed, characterized in that the stapling time estimating unit uses an arithmetic expression represented by Ts = Ts 0 + α * M *Tpa / Tp 0 where Ts is the estimated stapling processing time, Ts0 is a standard stapling processing time to perform predetermined stapling processing on one sheet of plain paper, a is a coefficient with respect to an increase in a stapling processing time per sheet, M is the number of sheets forming a sheet bundle, Tpa is an average value of the punching processing times, and Tp0 is a standard punching processing time to perform predetermined punching processing on one sheet of plain paper, and the stapling time estimating unit substitutes the average value Tpa of the punching processing times and the number of sheets M into the arithmetic expression, thereby calculating the estimated stapling processing time Ts. a post-processing device configured to perform punching processing on a sheet on which an image is formed and stapling processing on a sheet bundle consisting of a plurality of the punched sheets; a punching time measuring unit configured to measure a punching processing time to perform the punching processing; a stapling time estimating unit configured to calculate an estimated stapling processing time to perform stapling processing by using the punching processing time measured by the punching time measuring unit;and a sheet-bundle interval time setting unit configured to set, by using the estimated stapling processing time, a sheet-bundle interval time, which is a time from when the last sheet forming a sheet bundle subjected to the punching processing is fed until when the first sheet forming a subsequent sheet bundle subjected to the punching processing is fed, characterized in that the stapling time estimating unit uses an arithmetic expression represented by Ts = Ts 0 + α * M *Tpa / Tp 0 where Ts is the estimated stapling processing time, Ts0 is a standard stapling processing time to perform predetermined stapling processing on one sheet of plain paper, a is a coefficient with respect to an increase in a stapling processing time per sheet, M is the number of sheets forming a sheet bundle, Tpa is an average value of the punching processing times, and Tp0 is a standard punching processing time to perform predetermined punching processing on one sheet of plain paper, and the stapling time estimating unit substitutes the average value Tpa of the punching processing times and the number of sheets M into the arithmetic expression, thereby calculating the estimated stapling processing time Ts.