A motor drive device (12;32) for performing a positioning operation by driving a motor while performing feedback control so that actual operation follows a position command for commanding a position, the motor drive device comprising: a feedback controller for performing feedback control by generating a torque command based on a position deviation which is a difference between the position command and a detected position information, and outputting the torque command; a speed command calculator for calculating a speed command based on a temporal difference of the position command; a viscous friction calculator for calculating a viscous friction torque compensation value by multiplying the speed command by a viscous friction coefficient; characterized by an evaluation index measurement section for measuring an evaluation index of the position deviation until a positioning of the positioning operation is completed; a servo adjustment section for determining an optimum value of the viscous friction coefficient based on the evaluation index informed from the evaluation index measurement section, and setting the optimum value of the viscous friction coefficient with the viscous friction calculator; and an adder for adding the viscous friction torque compensation value to the torque command, and outputting as a driving signal to drive the motor, wherein the servo adjustment section repeats the positioning operation a plurality of times while changing the viscous friction coefficient, and obtains the evaluation index output from the evaluation index measurement section every time the positioning operation is performed; and the servo adjustment section determines the viscous friction coefficient as the optimum value when the obtained evaluation index becomes the specified value of the predetermined position deviation. a feedback controller for performing feedback control by generating a torque command based on a position deviation which is a difference between the position command and a detected position information, and outputting the torque command; a speed command calculator for calculating a speed command based on a temporal difference of the position command; a viscous friction calculator for calculating a viscous friction torque compensation value by multiplying the speed command by a viscous friction coefficient; an evaluation index measurement section for measuring an evaluation index of the position deviation until a positioning of the positioning operation is completed; a servo adjustment section for determining an optimum value of the viscous friction coefficient based on the evaluation index informed from the evaluation index measurement section, and setting the optimum value of the viscous friction coefficient with the viscous friction calculator; and an adder for adding the viscous friction torque compensation value to the torque command, and outputting as a driving signal to drive the motor, the servo adjustment section repeats the positioning operation a plurality of times while changing the viscous friction coefficient, and obtains the evaluation index output from the evaluation index measurement section every time the positioning operation is performed; and the servo adjustment section determines the viscous friction coefficient as the optimum value when the obtained evaluation index becomes the specified value of the predetermined position deviation.