In a sheet post-processing apparatus to perform a folding processing to a sheet bundle, a folding roller drive motor to drive a pair of folding rollers is PWM driven, and its speed is changed according to the sheet type or sheet size and between a period before a blade comes in contact with the sheet bundle, a period when the blade is being inserted between the folding rollers after coming in contact with the sheet bundle, and a period when folding is being performed by the folding rollers.