A fold-enforcing assembly comprising: a fold-enforcing device including: a pair of pressing members (17a,17b) configured to nip and press a fold of a sheet bundle in a direction of thickness of the sheet bundle; a pressing mechanism (18a,18b,19a,19b,21,22,23,24,25) configured to pressurize and depressurize the pair of pressing members (17a,17b) in the direction of thickness; and a driver configured to drive the pressing mechanism (18a,18b,19a,19b,21,22,23,24,25); a moving device configured to move the fold-enforcing device in a direction of the fold; and a control circuit (101,102) configured to: move, with the moving device, the fold-enforcing device in accordance with a size of the sheet bundle in the direction of the fold; pressurize, with the pressing mechanism (18a,18b,19a,19b,21,22,23,24,25), the pair of pressing members (17a,17b) in the direction of thickness, to press a first end portion of the sheet bundle in the direction of the fold; and move, with the moving device, the fold-enforcing device to a second end portion of the sheet bundle opposite the first end portion in the direction of the fold; characterized in that the control circuit (101,102) is further configured to: depressurize, with the pressing mechanism (18a,18b,19a,19b,21,22,23,24,25), the pair of pressing members (17a,17b) in the second end portion. a fold-enforcing device including: a pair of pressing members (17a,17b) configured to nip and press a fold of a sheet bundle in a direction of thickness of the sheet bundle; a pressing mechanism (18a,18b,19a,19b,21,22,23,24,25) configured to pressurize and depressurize the pair of pressing members (17a,17b) in the direction of thickness; and a driver configured to drive the pressing mechanism (18a,18b,19a,19b,21,22,23,24,25); a pair of pressing members (17a,17b) configured to nip and press a fold of a sheet bundle in a direction of thickness of the sheet bundle; a pressing mechanism (18a,18b,19a,19b,21,22,23,24,25) configured to pressurize and depressurize the pair of pressing members (17a,17b) in the direction of thickness; and a driver configured to drive the pressing mechanism (18a,18b,19a,19b,21,22,23,24,25); a moving device configured to move the fold-enforcing device in a direction of the fold; and a control circuit (101,102) configured to: move, with the moving device, the fold-enforcing device in accordance with a size of the sheet bundle in the direction of the fold; pressurize, with the pressing mechanism (18a,18b,19a,19b,21,22,23,24,25), the pair of pressing members (17a,17b) in the direction of thickness, to press a first end portion of the sheet bundle in the direction of the fold; and move, with the moving device, the fold-enforcing device to a second end portion of the sheet bundle opposite the first end portion in the direction of the fold; characterized in that the control circuit (101,102) is further configured to: depressurize, with the pressing mechanism (18a,18b,19a,19b,21,22,23,24,25), the pair of pressing members (17a,17b) in the second end portion. move, with the moving device, the fold-enforcing device in accordance with a size of the sheet bundle in the direction of the fold; pressurize, with the pressing mechanism (18a,18b,19a,19b,21,22,23,24,25), the pair of pressing members (17a,17b) in the direction of thickness, to press a first end portion of the sheet bundle in the direction of the fold; and move, with the moving device, the fold-enforcing device to a second end portion of the sheet bundle opposite the first end portion in the direction of the fold; characterized in that the control circuit (101,102) is further configured to: depressurize, with the pressing mechanism (18a,18b,19a,19b,21,22,23,24,25), the pair of pressing members (17a,17b) in the second end portion.