There is provided a technique in which in a case where a sheet folding process or a staple process is performed, even in the case where the size precision of a sheet used is not satisfactory, the high-precision staple process and folding process can be performed to the sheet. In a sheet processing method of a sheet processing apparatus to perform a specified process to a sheet, a relatively moved sheet is detected, the size of the sheet is calculated based on the detection result, and a position where the specified process is performed to a bundle of sheets as an object of the specified process is adjusted based on the calculated sheet size.