A method of optimal focusing for a document scanner is disclosed. The scanner includes an optic module movable in a given scanning direction to perform scanning operation over an area of a document in a scan line by scan line manner by being driven by an optic module moving mechanism under control of a control unit. The control unit includes a best focus value memory for storage of the best focus value for each scan line. The method includes the steps of moving the optic module to one of scan lines of the document and retrieving the best focus value corresponding to the selected scan line. A focus adjusting mechanism is controlled by the control unit to move the optic module in a second direction to adjust the focus position of the optic module in accordance with the best focus value of the scan line. Once the optic module reaches the scan line, the operation of the optic module is temporarily suspended in order to have the vibration of the optic module damped out. Thereafter, a scanning operation is performed over the scan line.